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ll_rw_blk.c

/*
 * Copyright (C) 1991, 1992 Linus Torvalds
 * Copyright (C) 1994,      Karl Keyte: Added support for disk statistics
 * Elevator latency, (C) 2000  Andrea Arcangeli <andrea@suse.de> SuSE
 * Queue request tables / lock, selectable elevator, Jens Axboe <axboe@suse.de>
 * kernel-doc documentation started by NeilBrown <neilb@cse.unsw.edu.au> -  July2000
 * bio rewrite, highmem i/o, etc, Jens Axboe <axboe@suse.de> - may 2001
 */

/*
 * This handles all read/write requests to block devices
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/backing-dev.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/mm.h>
#include <linux/kernel_stat.h>
#include <linux/string.h>
#include <linux/init.h>
#include <linux/bootmem.h>    /* for max_pfn/max_low_pfn */
#include <linux/completion.h>
#include <linux/slab.h>
#include <linux/swap.h>
#include <linux/writeback.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/interrupt.h>
#include <linux/cpu.h>
#include <linux/blktrace_api.h>
#include <linux/fault-inject.h>
#include <linux/scatterlist.h>

/*
 * for max sense size
 */
#include <scsi/scsi_cmnd.h>

static void blk_unplug_work(struct work_struct *work);
static void blk_unplug_timeout(unsigned long data);
static void drive_stat_acct(struct request *rq, int new_io);
static void init_request_from_bio(struct request *req, struct bio *bio);
static int __make_request(struct request_queue *q, struct bio *bio);
static struct io_context *current_io_context(gfp_t gfp_flags, int node);
static void blk_recalc_rq_segments(struct request *rq);
static void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
                      struct bio *bio);

/*
 * For the allocated request tables
 */
static struct kmem_cache *request_cachep;

/*
 * For queue allocation
 */
static struct kmem_cache *requestq_cachep;

/*
 * For io context allocations
 */
static struct kmem_cache *iocontext_cachep;

/*
 * Controlling structure to kblockd
 */
static struct workqueue_struct *kblockd_workqueue;

unsigned long blk_max_low_pfn, blk_max_pfn;

EXPORT_SYMBOL(blk_max_low_pfn);
EXPORT_SYMBOL(blk_max_pfn);

static DEFINE_PER_CPU(struct list_head, blk_cpu_done);

/* Amount of time in which a process may batch requests */
#define BLK_BATCH_TIME  (HZ/50UL)

/* Number of requests a "batching" process may submit */
#define BLK_BATCH_REQ   32

/*
 * Return the threshold (number of used requests) at which the queue is
 * considered to be congested.  It include a little hysteresis to keep the
 * context switch rate down.
 */
static inline int queue_congestion_on_threshold(struct request_queue *q)
{
      return q->nr_congestion_on;
}

/*
 * The threshold at which a queue is considered to be uncongested
 */
static inline int queue_congestion_off_threshold(struct request_queue *q)
{
      return q->nr_congestion_off;
}

static void blk_queue_congestion_threshold(struct request_queue *q)
{
      int nr;

      nr = q->nr_requests - (q->nr_requests / 8) + 1;
      if (nr > q->nr_requests)
            nr = q->nr_requests;
      q->nr_congestion_on = nr;

      nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests / 16) - 1;
      if (nr < 1)
            nr = 1;
      q->nr_congestion_off = nr;
}

/**
 * blk_get_backing_dev_info - get the address of a queue's backing_dev_info
 * @bdev:   device
 *
 * Locates the passed device's request queue and returns the address of its
 * backing_dev_info
 *
 * Will return NULL if the request queue cannot be located.
 */
struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev)
{
      struct backing_dev_info *ret = NULL;
      struct request_queue *q = bdev_get_queue(bdev);

      if (q)
            ret = &q->backing_dev_info;
      return ret;
}
EXPORT_SYMBOL(blk_get_backing_dev_info);

/**
 * blk_queue_prep_rq - set a prepare_request function for queue
 * @q:            queue
 * @pfn:    prepare_request function
 *
 * It's possible for a queue to register a prepare_request callback which
 * is invoked before the request is handed to the request_fn. The goal of
 * the function is to prepare a request for I/O, it can be used to build a
 * cdb from the request data for instance.
 *
 */
void blk_queue_prep_rq(struct request_queue *q, prep_rq_fn *pfn)
{
      q->prep_rq_fn = pfn;
}

EXPORT_SYMBOL(blk_queue_prep_rq);

/**
 * blk_queue_merge_bvec - set a merge_bvec function for queue
 * @q:            queue
 * @mbfn:   merge_bvec_fn
 *
 * Usually queues have static limitations on the max sectors or segments that
 * we can put in a request. Stacking drivers may have some settings that
 * are dynamic, and thus we have to query the queue whether it is ok to
 * add a new bio_vec to a bio at a given offset or not. If the block device
 * has such limitations, it needs to register a merge_bvec_fn to control
 * the size of bio's sent to it. Note that a block device *must* allow a
 * single page to be added to an empty bio. The block device driver may want
 * to use the bio_split() function to deal with these bio's. By default
 * no merge_bvec_fn is defined for a queue, and only the fixed limits are
 * honored.
 */
void blk_queue_merge_bvec(struct request_queue *q, merge_bvec_fn *mbfn)
{
      q->merge_bvec_fn = mbfn;
}

EXPORT_SYMBOL(blk_queue_merge_bvec);

void blk_queue_softirq_done(struct request_queue *q, softirq_done_fn *fn)
{
      q->softirq_done_fn = fn;
}

EXPORT_SYMBOL(blk_queue_softirq_done);

/**
 * blk_queue_make_request - define an alternate make_request function for a device
 * @q:  the request queue for the device to be affected
 * @mfn: the alternate make_request function
 *
 * Description:
 *    The normal way for &struct bios to be passed to a device
 *    driver is for them to be collected into requests on a request
 *    queue, and then to allow the device driver to select requests
 *    off that queue when it is ready.  This works well for many block
 *    devices. However some block devices (typically virtual devices
 *    such as md or lvm) do not benefit from the processing on the
 *    request queue, and are served best by having the requests passed
 *    directly to them.  This can be achieved by providing a function
 *    to blk_queue_make_request().
 *
 * Caveat:
 *    The driver that does this *must* be able to deal appropriately
 *    with buffers in "highmemory". This can be accomplished by either calling
 *    __bio_kmap_atomic() to get a temporary kernel mapping, or by calling
 *    blk_queue_bounce() to create a buffer in normal memory.
 **/
void blk_queue_make_request(struct request_queue * q, make_request_fn * mfn)
{
      /*
       * set defaults
       */
      q->nr_requests = BLKDEV_MAX_RQ;
      blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
      blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
      q->make_request_fn = mfn;
      q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
      q->backing_dev_info.state = 0;
      q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
      blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
      blk_queue_hardsect_size(q, 512);
      blk_queue_dma_alignment(q, 511);
      blk_queue_congestion_threshold(q);
      q->nr_batching = BLK_BATCH_REQ;

      q->unplug_thresh = 4;         /* hmm */
      q->unplug_delay = (3 * HZ) / 1000;  /* 3 milliseconds */
      if (q->unplug_delay == 0)
            q->unplug_delay = 1;

      INIT_WORK(&q->unplug_work, blk_unplug_work);

      q->unplug_timer.function = blk_unplug_timeout;
      q->unplug_timer.data = (unsigned long)q;

      /*
       * by default assume old behaviour and bounce for any highmem page
       */
      blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH);
}

EXPORT_SYMBOL(blk_queue_make_request);

static void rq_init(struct request_queue *q, struct request *rq)
{
      INIT_LIST_HEAD(&rq->queuelist);
      INIT_LIST_HEAD(&rq->donelist);

      rq->errors = 0;
      rq->bio = rq->biotail = NULL;
      INIT_HLIST_NODE(&rq->hash);
      RB_CLEAR_NODE(&rq->rb_node);
      rq->ioprio = 0;
      rq->buffer = NULL;
      rq->ref_count = 1;
      rq->q = q;
      rq->special = NULL;
      rq->data_len = 0;
      rq->data = NULL;
      rq->nr_phys_segments = 0;
      rq->sense = NULL;
      rq->end_io = NULL;
      rq->end_io_data = NULL;
      rq->completion_data = NULL;
      rq->next_rq = NULL;
}

/**
 * blk_queue_ordered - does this queue support ordered writes
 * @q:        the request queue
 * @ordered:  one of QUEUE_ORDERED_*
 * @prepare_flush_fn: rq setup helper for cache flush ordered writes
 *
 * Description:
 *   For journalled file systems, doing ordered writes on a commit
 *   block instead of explicitly doing wait_on_buffer (which is bad
 *   for performance) can be a big win. Block drivers supporting this
 *   feature should call this function and indicate so.
 *
 **/
int blk_queue_ordered(struct request_queue *q, unsigned ordered,
                  prepare_flush_fn *prepare_flush_fn)
{
      if (ordered & (QUEUE_ORDERED_PREFLUSH | QUEUE_ORDERED_POSTFLUSH) &&
          prepare_flush_fn == NULL) {
            printk(KERN_ERR "blk_queue_ordered: prepare_flush_fn required\n");
            return -EINVAL;
      }

      if (ordered != QUEUE_ORDERED_NONE &&
          ordered != QUEUE_ORDERED_DRAIN &&
          ordered != QUEUE_ORDERED_DRAIN_FLUSH &&
          ordered != QUEUE_ORDERED_DRAIN_FUA &&
          ordered != QUEUE_ORDERED_TAG &&
          ordered != QUEUE_ORDERED_TAG_FLUSH &&
          ordered != QUEUE_ORDERED_TAG_FUA) {
            printk(KERN_ERR "blk_queue_ordered: bad value %d\n", ordered);
            return -EINVAL;
      }

      q->ordered = ordered;
      q->next_ordered = ordered;
      q->prepare_flush_fn = prepare_flush_fn;

      return 0;
}

EXPORT_SYMBOL(blk_queue_ordered);

/*
 * Cache flushing for ordered writes handling
 */
inline unsigned blk_ordered_cur_seq(struct request_queue *q)
{
      if (!q->ordseq)
            return 0;
      return 1 << ffz(q->ordseq);
}

unsigned blk_ordered_req_seq(struct request *rq)
{
      struct request_queue *q = rq->q;

      BUG_ON(q->ordseq == 0);

      if (rq == &q->pre_flush_rq)
            return QUEUE_ORDSEQ_PREFLUSH;
      if (rq == &q->bar_rq)
            return QUEUE_ORDSEQ_BAR;
      if (rq == &q->post_flush_rq)
            return QUEUE_ORDSEQ_POSTFLUSH;

      /*
       * !fs requests don't need to follow barrier ordering.  Always
       * put them at the front.  This fixes the following deadlock.
       *
       * http://thread.gmane.org/gmane.linux.kernel/537473
       */
      if (!blk_fs_request(rq))
            return QUEUE_ORDSEQ_DRAIN;

      if ((rq->cmd_flags & REQ_ORDERED_COLOR) ==
          (q->orig_bar_rq->cmd_flags & REQ_ORDERED_COLOR))
            return QUEUE_ORDSEQ_DRAIN;
      else
            return QUEUE_ORDSEQ_DONE;
}

void blk_ordered_complete_seq(struct request_queue *q, unsigned seq, int error)
{
      struct request *rq;
      int uptodate;

      if (error && !q->orderr)
            q->orderr = error;

      BUG_ON(q->ordseq & seq);
      q->ordseq |= seq;

      if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE)
            return;

      /*
       * Okay, sequence complete.
       */
      uptodate = 1;
      if (q->orderr)
            uptodate = q->orderr;

      q->ordseq = 0;
      rq = q->orig_bar_rq;

      end_that_request_first(rq, uptodate, rq->hard_nr_sectors);
      end_that_request_last(rq, uptodate);
}

static void pre_flush_end_io(struct request *rq, int error)
{
      elv_completed_request(rq->q, rq);
      blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_PREFLUSH, error);
}

static void bar_end_io(struct request *rq, int error)
{
      elv_completed_request(rq->q, rq);
      blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_BAR, error);
}

static void post_flush_end_io(struct request *rq, int error)
{
      elv_completed_request(rq->q, rq);
      blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_POSTFLUSH, error);
}

static void queue_flush(struct request_queue *q, unsigned which)
{
      struct request *rq;
      rq_end_io_fn *end_io;

      if (which == QUEUE_ORDERED_PREFLUSH) {
            rq = &q->pre_flush_rq;
            end_io = pre_flush_end_io;
      } else {
            rq = &q->post_flush_rq;
            end_io = post_flush_end_io;
      }

      rq->cmd_flags = REQ_HARDBARRIER;
      rq_init(q, rq);
      rq->elevator_private = NULL;
      rq->elevator_private2 = NULL;
      rq->rq_disk = q->bar_rq.rq_disk;
      rq->end_io = end_io;
      q->prepare_flush_fn(q, rq);

      elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
}

static inline struct request *start_ordered(struct request_queue *q,
                                  struct request *rq)
{
      q->orderr = 0;
      q->ordered = q->next_ordered;
      q->ordseq |= QUEUE_ORDSEQ_STARTED;

      /*
       * Prep proxy barrier request.
       */
      blkdev_dequeue_request(rq);
      q->orig_bar_rq = rq;
      rq = &q->bar_rq;
      rq->cmd_flags = 0;
      rq_init(q, rq);
      if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
            rq->cmd_flags |= REQ_RW;
      if (q->ordered & QUEUE_ORDERED_FUA)
            rq->cmd_flags |= REQ_FUA;
      rq->elevator_private = NULL;
      rq->elevator_private2 = NULL;
      init_request_from_bio(rq, q->orig_bar_rq->bio);
      rq->end_io = bar_end_io;

      /*
       * Queue ordered sequence.  As we stack them at the head, we
       * need to queue in reverse order.  Note that we rely on that
       * no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
       * request gets inbetween ordered sequence. If this request is
       * an empty barrier, we don't need to do a postflush ever since
       * there will be no data written between the pre and post flush.
       * Hence a single flush will suffice.
       */
      if ((q->ordered & QUEUE_ORDERED_POSTFLUSH) && !blk_empty_barrier(rq))
            queue_flush(q, QUEUE_ORDERED_POSTFLUSH);
      else
            q->ordseq |= QUEUE_ORDSEQ_POSTFLUSH;

      elv_insert(q, rq, ELEVATOR_INSERT_FRONT);

      if (q->ordered & QUEUE_ORDERED_PREFLUSH) {
            queue_flush(q, QUEUE_ORDERED_PREFLUSH);
            rq = &q->pre_flush_rq;
      } else
            q->ordseq |= QUEUE_ORDSEQ_PREFLUSH;

      if ((q->ordered & QUEUE_ORDERED_TAG) || q->in_flight == 0)
            q->ordseq |= QUEUE_ORDSEQ_DRAIN;
      else
            rq = NULL;

      return rq;
}

int blk_do_ordered(struct request_queue *q, struct request **rqp)
{
      struct request *rq = *rqp;
      const int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);

      if (!q->ordseq) {
            if (!is_barrier)
                  return 1;

            if (q->next_ordered != QUEUE_ORDERED_NONE) {
                  *rqp = start_ordered(q, rq);
                  return 1;
            } else {
                  /*
                   * This can happen when the queue switches to
                   * ORDERED_NONE while this request is on it.
                   */
                  blkdev_dequeue_request(rq);
                  end_that_request_first(rq, -EOPNOTSUPP,
                                     rq->hard_nr_sectors);
                  end_that_request_last(rq, -EOPNOTSUPP);
                  *rqp = NULL;
                  return 0;
            }
      }

      /*
       * Ordered sequence in progress
       */

      /* Special requests are not subject to ordering rules. */
      if (!blk_fs_request(rq) &&
          rq != &q->pre_flush_rq && rq != &q->post_flush_rq)
            return 1;

      if (q->ordered & QUEUE_ORDERED_TAG) {
            /* Ordered by tag.  Blocking the next barrier is enough. */
            if (is_barrier && rq != &q->bar_rq)
                  *rqp = NULL;
      } else {
            /* Ordered by draining.  Wait for turn. */
            WARN_ON(blk_ordered_req_seq(rq) < blk_ordered_cur_seq(q));
            if (blk_ordered_req_seq(rq) > blk_ordered_cur_seq(q))
                  *rqp = NULL;
      }

      return 1;
}

static void req_bio_endio(struct request *rq, struct bio *bio,
                    unsigned int nbytes, int error)
{
      struct request_queue *q = rq->q;

      if (&q->bar_rq != rq) {
            if (error)
                  clear_bit(BIO_UPTODATE, &bio->bi_flags);
            else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
                  error = -EIO;

            if (unlikely(nbytes > bio->bi_size)) {
                  printk("%s: want %u bytes done, only %u left\n",
                         __FUNCTION__, nbytes, bio->bi_size);
                  nbytes = bio->bi_size;
            }

            bio->bi_size -= nbytes;
            bio->bi_sector += (nbytes >> 9);
            if (bio->bi_size == 0)
                  bio_endio(bio, error);
      } else {

            /*
             * Okay, this is the barrier request in progress, just
             * record the error;
             */
            if (error && !q->orderr)
                  q->orderr = error;
      }
}

/**
 * blk_queue_bounce_limit - set bounce buffer limit for queue
 * @q:  the request queue for the device
 * @dma_addr:   bus address limit
 *
 * Description:
 *    Different hardware can have different requirements as to what pages
 *    it can do I/O directly to. A low level driver can call
 *    blk_queue_bounce_limit to have lower memory pages allocated as bounce
 *    buffers for doing I/O to pages residing above @page.
 **/
void blk_queue_bounce_limit(struct request_queue *q, u64 dma_addr)
{
      unsigned long bounce_pfn = dma_addr >> PAGE_SHIFT;
      int dma = 0;

      q->bounce_gfp = GFP_NOIO;
#if BITS_PER_LONG == 64
      /* Assume anything <= 4GB can be handled by IOMMU.
         Actually some IOMMUs can handle everything, but I don't
         know of a way to test this here. */
      if (bounce_pfn < (min_t(u64,0xffffffff,BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
            dma = 1;
      q->bounce_pfn = max_low_pfn;
#else
      if (bounce_pfn < blk_max_low_pfn)
            dma = 1;
      q->bounce_pfn = bounce_pfn;
#endif
      if (dma) {
            init_emergency_isa_pool();
            q->bounce_gfp = GFP_NOIO | GFP_DMA;
            q->bounce_pfn = bounce_pfn;
      }
}

EXPORT_SYMBOL(blk_queue_bounce_limit);

/**
 * blk_queue_max_sectors - set max sectors for a request for this queue
 * @q:  the request queue for the device
 * @max_sectors:  max sectors in the usual 512b unit
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the size of
 *    received requests.
 **/
void blk_queue_max_sectors(struct request_queue *q, unsigned int max_sectors)
{
      if ((max_sectors << 9) < PAGE_CACHE_SIZE) {
            max_sectors = 1 << (PAGE_CACHE_SHIFT - 9);
            printk("%s: set to minimum %d\n", __FUNCTION__, max_sectors);
      }

      if (BLK_DEF_MAX_SECTORS > max_sectors)
            q->max_hw_sectors = q->max_sectors = max_sectors;
      else {
            q->max_sectors = BLK_DEF_MAX_SECTORS;
            q->max_hw_sectors = max_sectors;
      }
}

EXPORT_SYMBOL(blk_queue_max_sectors);

/**
 * blk_queue_max_phys_segments - set max phys segments for a request for this queue
 * @q:  the request queue for the device
 * @max_segments:  max number of segments
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the number of
 *    physical data segments in a request.  This would be the largest sized
 *    scatter list the driver could handle.
 **/
void blk_queue_max_phys_segments(struct request_queue *q,
                         unsigned short max_segments)
{
      if (!max_segments) {
            max_segments = 1;
            printk("%s: set to minimum %d\n", __FUNCTION__, max_segments);
      }

      q->max_phys_segments = max_segments;
}

EXPORT_SYMBOL(blk_queue_max_phys_segments);

/**
 * blk_queue_max_hw_segments - set max hw segments for a request for this queue
 * @q:  the request queue for the device
 * @max_segments:  max number of segments
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the number of
 *    hw data segments in a request.  This would be the largest number of
 *    address/length pairs the host adapter can actually give as once
 *    to the device.
 **/
void blk_queue_max_hw_segments(struct request_queue *q,
                         unsigned short max_segments)
{
      if (!max_segments) {
            max_segments = 1;
            printk("%s: set to minimum %d\n", __FUNCTION__, max_segments);
      }

      q->max_hw_segments = max_segments;
}

EXPORT_SYMBOL(blk_queue_max_hw_segments);

/**
 * blk_queue_max_segment_size - set max segment size for blk_rq_map_sg
 * @q:  the request queue for the device
 * @max_size:  max size of segment in bytes
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the size of a
 *    coalesced segment
 **/
void blk_queue_max_segment_size(struct request_queue *q, unsigned int max_size)
{
      if (max_size < PAGE_CACHE_SIZE) {
            max_size = PAGE_CACHE_SIZE;
            printk("%s: set to minimum %d\n", __FUNCTION__, max_size);
      }

      q->max_segment_size = max_size;
}

EXPORT_SYMBOL(blk_queue_max_segment_size);

/**
 * blk_queue_hardsect_size - set hardware sector size for the queue
 * @q:  the request queue for the device
 * @size:  the hardware sector size, in bytes
 *
 * Description:
 *   This should typically be set to the lowest possible sector size
 *   that the hardware can operate on (possible without reverting to
 *   even internal read-modify-write operations). Usually the default
 *   of 512 covers most hardware.
 **/
void blk_queue_hardsect_size(struct request_queue *q, unsigned short size)
{
      q->hardsect_size = size;
}

EXPORT_SYMBOL(blk_queue_hardsect_size);

/*
 * Returns the minimum that is _not_ zero, unless both are zero.
 */
#define min_not_zero(l, r) (l == 0) ? r : ((r == 0) ? l : min(l, r))

/**
 * blk_queue_stack_limits - inherit underlying queue limits for stacked drivers
 * @t:      the stacking driver (top)
 * @b:  the underlying device (bottom)
 **/
void blk_queue_stack_limits(struct request_queue *t, struct request_queue *b)
{
      /* zero is "infinity" */
      t->max_sectors = min_not_zero(t->max_sectors,b->max_sectors);
      t->max_hw_sectors = min_not_zero(t->max_hw_sectors,b->max_hw_sectors);

      t->max_phys_segments = min(t->max_phys_segments,b->max_phys_segments);
      t->max_hw_segments = min(t->max_hw_segments,b->max_hw_segments);
      t->max_segment_size = min(t->max_segment_size,b->max_segment_size);
      t->hardsect_size = max(t->hardsect_size,b->hardsect_size);
      if (!test_bit(QUEUE_FLAG_CLUSTER, &b->queue_flags))
            clear_bit(QUEUE_FLAG_CLUSTER, &t->queue_flags);
}

EXPORT_SYMBOL(blk_queue_stack_limits);

/**
 * blk_queue_segment_boundary - set boundary rules for segment merging
 * @q:  the request queue for the device
 * @mask:  the memory boundary mask
 **/
void blk_queue_segment_boundary(struct request_queue *q, unsigned long mask)
{
      if (mask < PAGE_CACHE_SIZE - 1) {
            mask = PAGE_CACHE_SIZE - 1;
            printk("%s: set to minimum %lx\n", __FUNCTION__, mask);
      }

      q->seg_boundary_mask = mask;
}

EXPORT_SYMBOL(blk_queue_segment_boundary);

/**
 * blk_queue_dma_alignment - set dma length and memory alignment
 * @q:     the request queue for the device
 * @mask:  alignment mask
 *
 * description:
 *    set required memory and length aligment for direct dma transactions.
 *    this is used when buiding direct io requests for the queue.
 *
 **/
void blk_queue_dma_alignment(struct request_queue *q, int mask)
{
      q->dma_alignment = mask;
}

EXPORT_SYMBOL(blk_queue_dma_alignment);

/**
 * blk_queue_find_tag - find a request by its tag and queue
 * @q:       The request queue for the device
 * @tag: The tag of the request
 *
 * Notes:
 *    Should be used when a device returns a tag and you want to match
 *    it with a request.
 *
 *    no locks need be held.
 **/
struct request *blk_queue_find_tag(struct request_queue *q, int tag)
{
      return blk_map_queue_find_tag(q->queue_tags, tag);
}

EXPORT_SYMBOL(blk_queue_find_tag);

/**
 * __blk_free_tags - release a given set of tag maintenance info
 * @bqt:    the tag map to free
 *
 * Tries to free the specified @bqt@.  Returns true if it was
 * actually freed and false if there are still references using it
 */
static int __blk_free_tags(struct blk_queue_tag *bqt)
{
      int retval;

      retval = atomic_dec_and_test(&bqt->refcnt);
      if (retval) {
            BUG_ON(bqt->busy);

            kfree(bqt->tag_index);
            bqt->tag_index = NULL;

            kfree(bqt->tag_map);
            bqt->tag_map = NULL;

            kfree(bqt);

      }

      return retval;
}

/**
 * __blk_queue_free_tags - release tag maintenance info
 * @q:  the request queue for the device
 *
 *  Notes:
 *    blk_cleanup_queue() will take care of calling this function, if tagging
 *    has been used. So there's no need to call this directly.
 **/
static void __blk_queue_free_tags(struct request_queue *q)
{
      struct blk_queue_tag *bqt = q->queue_tags;

      if (!bqt)
            return;

      __blk_free_tags(bqt);

      q->queue_tags = NULL;
      q->queue_flags &= ~(1 << QUEUE_FLAG_QUEUED);
}


/**
 * blk_free_tags - release a given set of tag maintenance info
 * @bqt:    the tag map to free
 *
 * For externally managed @bqt@ frees the map.  Callers of this
 * function must guarantee to have released all the queues that
 * might have been using this tag map.
 */
void blk_free_tags(struct blk_queue_tag *bqt)
{
      if (unlikely(!__blk_free_tags(bqt)))
            BUG();
}
EXPORT_SYMBOL(blk_free_tags);

/**
 * blk_queue_free_tags - release tag maintenance info
 * @q:  the request queue for the device
 *
 *  Notes:
 *    This is used to disabled tagged queuing to a device, yet leave
 *    queue in function.
 **/
void blk_queue_free_tags(struct request_queue *q)
{
      clear_bit(QUEUE_FLAG_QUEUED, &q->queue_flags);
}

EXPORT_SYMBOL(blk_queue_free_tags);

static int
init_tag_map(struct request_queue *q, struct blk_queue_tag *tags, int depth)
{
      struct request **tag_index;
      unsigned long *tag_map;
      int nr_ulongs;

      if (q && depth > q->nr_requests * 2) {
            depth = q->nr_requests * 2;
            printk(KERN_ERR "%s: adjusted depth to %d\n",
                        __FUNCTION__, depth);
      }

      tag_index = kzalloc(depth * sizeof(struct request *), GFP_ATOMIC);
      if (!tag_index)
            goto fail;

      nr_ulongs = ALIGN(depth, BITS_PER_LONG) / BITS_PER_LONG;
      tag_map = kzalloc(nr_ulongs * sizeof(unsigned long), GFP_ATOMIC);
      if (!tag_map)
            goto fail;

      tags->real_max_depth = depth;
      tags->max_depth = depth;
      tags->tag_index = tag_index;
      tags->tag_map = tag_map;

      return 0;
fail:
      kfree(tag_index);
      return -ENOMEM;
}

static struct blk_queue_tag *__blk_queue_init_tags(struct request_queue *q,
                                       int depth)
{
      struct blk_queue_tag *tags;

      tags = kmalloc(sizeof(struct blk_queue_tag), GFP_ATOMIC);
      if (!tags)
            goto fail;

      if (init_tag_map(q, tags, depth))
            goto fail;

      tags->busy = 0;
      atomic_set(&tags->refcnt, 1);
      return tags;
fail:
      kfree(tags);
      return NULL;
}

/**
 * blk_init_tags - initialize the tag info for an external tag map
 * @depth:  the maximum queue depth supported
 * @tags: the tag to use
 **/
struct blk_queue_tag *blk_init_tags(int depth)
{
      return __blk_queue_init_tags(NULL, depth);
}
EXPORT_SYMBOL(blk_init_tags);

/**
 * blk_queue_init_tags - initialize the queue tag info
 * @q:  the request queue for the device
 * @depth:  the maximum queue depth supported
 * @tags: the tag to use
 **/
int blk_queue_init_tags(struct request_queue *q, int depth,
                  struct blk_queue_tag *tags)
{
      int rc;

      BUG_ON(tags && q->queue_tags && tags != q->queue_tags);

      if (!tags && !q->queue_tags) {
            tags = __blk_queue_init_tags(q, depth);

            if (!tags)
                  goto fail;
      } else if (q->queue_tags) {
            if ((rc = blk_queue_resize_tags(q, depth)))
                  return rc;
            set_bit(QUEUE_FLAG_QUEUED, &q->queue_flags);
            return 0;
      } else
            atomic_inc(&tags->refcnt);

      /*
       * assign it, all done
       */
      q->queue_tags = tags;
      q->queue_flags |= (1 << QUEUE_FLAG_QUEUED);
      INIT_LIST_HEAD(&q->tag_busy_list);
      return 0;
fail:
      kfree(tags);
      return -ENOMEM;
}

EXPORT_SYMBOL(blk_queue_init_tags);

/**
 * blk_queue_resize_tags - change the queueing depth
 * @q:  the request queue for the device
 * @new_depth: the new max command queueing depth
 *
 *  Notes:
 *    Must be called with the queue lock held.
 **/
int blk_queue_resize_tags(struct request_queue *q, int new_depth)
{
      struct blk_queue_tag *bqt = q->queue_tags;
      struct request **tag_index;
      unsigned long *tag_map;
      int max_depth, nr_ulongs;

      if (!bqt)
            return -ENXIO;

      /*
       * if we already have large enough real_max_depth.  just
       * adjust max_depth.  *NOTE* as requests with tag value
       * between new_depth and real_max_depth can be in-flight, tag
       * map can not be shrunk blindly here.
       */
      if (new_depth <= bqt->real_max_depth) {
            bqt->max_depth = new_depth;
            return 0;
      }

      /*
       * Currently cannot replace a shared tag map with a new
       * one, so error out if this is the case
       */
      if (atomic_read(&bqt->refcnt) != 1)
            return -EBUSY;

      /*
       * save the old state info, so we can copy it back
       */
      tag_index = bqt->tag_index;
      tag_map = bqt->tag_map;
      max_depth = bqt->real_max_depth;

      if (init_tag_map(q, bqt, new_depth))
            return -ENOMEM;

      memcpy(bqt->tag_index, tag_index, max_depth * sizeof(struct request *));
      nr_ulongs = ALIGN(max_depth, BITS_PER_LONG) / BITS_PER_LONG;
      memcpy(bqt->tag_map, tag_map, nr_ulongs * sizeof(unsigned long));

      kfree(tag_index);
      kfree(tag_map);
      return 0;
}

EXPORT_SYMBOL(blk_queue_resize_tags);

/**
 * blk_queue_end_tag - end tag operations for a request
 * @q:  the request queue for the device
 * @rq: the request that has completed
 *
 *  Description:
 *    Typically called when end_that_request_first() returns 0, meaning
 *    all transfers have been done for a request. It's important to call
 *    this function before end_that_request_last(), as that will put the
 *    request back on the free list thus corrupting the internal tag list.
 *
 *  Notes:
 *   queue lock must be held.
 **/
void blk_queue_end_tag(struct request_queue *q, struct request *rq)
{
      struct blk_queue_tag *bqt = q->queue_tags;
      int tag = rq->tag;

      BUG_ON(tag == -1);

      if (unlikely(tag >= bqt->real_max_depth))
            /*
             * This can happen after tag depth has been reduced.
             * FIXME: how about a warning or info message here?
             */
            return;

      list_del_init(&rq->queuelist);
      rq->cmd_flags &= ~REQ_QUEUED;
      rq->tag = -1;

      if (unlikely(bqt->tag_index[tag] == NULL))
            printk(KERN_ERR "%s: tag %d is missing\n",
                   __FUNCTION__, tag);

      bqt->tag_index[tag] = NULL;

      if (unlikely(!test_bit(tag, bqt->tag_map))) {
            printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
                   __FUNCTION__, tag);
            return;
      }
      /*
       * The tag_map bit acts as a lock for tag_index[bit], so we need
       * unlock memory barrier semantics.
       */
      clear_bit_unlock(tag, bqt->tag_map);
      bqt->busy--;
}

EXPORT_SYMBOL(blk_queue_end_tag);

/**
 * blk_queue_start_tag - find a free tag and assign it
 * @q:  the request queue for the device
 * @rq:  the block request that needs tagging
 *
 *  Description:
 *    This can either be used as a stand-alone helper, or possibly be
 *    assigned as the queue &prep_rq_fn (in which case &struct request
 *    automagically gets a tag assigned). Note that this function
 *    assumes that any type of request can be queued! if this is not
 *    true for your device, you must check the request type before
 *    calling this function.  The request will also be removed from
 *    the request queue, so it's the drivers responsibility to readd
 *    it if it should need to be restarted for some reason.
 *
 *  Notes:
 *   queue lock must be held.
 **/
int blk_queue_start_tag(struct request_queue *q, struct request *rq)
{
      struct blk_queue_tag *bqt = q->queue_tags;
      int tag;

      if (unlikely((rq->cmd_flags & REQ_QUEUED))) {
            printk(KERN_ERR 
                   "%s: request %p for device [%s] already tagged %d",
                   __FUNCTION__, rq,
                   rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->tag);
            BUG();
      }

      /*
       * Protect against shared tag maps, as we may not have exclusive
       * access to the tag map.
       */
      do {
            tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
            if (tag >= bqt->max_depth)
                  return 1;

      } while (test_and_set_bit_lock(tag, bqt->tag_map));
      /*
       * We need lock ordering semantics given by test_and_set_bit_lock.
       * See blk_queue_end_tag for details.
       */

      rq->cmd_flags |= REQ_QUEUED;
      rq->tag = tag;
      bqt->tag_index[tag] = rq;
      blkdev_dequeue_request(rq);
      list_add(&rq->queuelist, &q->tag_busy_list);
      bqt->busy++;
      return 0;
}

EXPORT_SYMBOL(blk_queue_start_tag);

/**
 * blk_queue_invalidate_tags - invalidate all pending tags
 * @q:  the request queue for the device
 *
 *  Description:
 *   Hardware conditions may dictate a need to stop all pending requests.
 *   In this case, we will safely clear the block side of the tag queue and
 *   readd all requests to the request queue in the right order.
 *
 *  Notes:
 *   queue lock must be held.
 **/
void blk_queue_invalidate_tags(struct request_queue *q)
{
      struct list_head *tmp, *n;

      list_for_each_safe(tmp, n, &q->tag_busy_list)
            blk_requeue_request(q, list_entry_rq(tmp));
}

EXPORT_SYMBOL(blk_queue_invalidate_tags);

void blk_dump_rq_flags(struct request *rq, char *msg)
{
      int bit;

      printk("%s: dev %s: type=%x, flags=%x\n", msg,
            rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->cmd_type,
            rq->cmd_flags);

      printk("\nsector %llu, nr/cnr %lu/%u\n", (unsigned long long)rq->sector,
                                           rq->nr_sectors,
                                           rq->current_nr_sectors);
      printk("bio %p, biotail %p, buffer %p, data %p, len %u\n", rq->bio, rq->biotail, rq->buffer, rq->data, rq->data_len);

      if (blk_pc_request(rq)) {
            printk("cdb: ");
            for (bit = 0; bit < sizeof(rq->cmd); bit++)
                  printk("%02x ", rq->cmd[bit]);
            printk("\n");
      }
}

EXPORT_SYMBOL(blk_dump_rq_flags);

void blk_recount_segments(struct request_queue *q, struct bio *bio)
{
      struct request rq;
      struct bio *nxt = bio->bi_next;
      rq.q = q;
      rq.bio = rq.biotail = bio;
      bio->bi_next = NULL;
      blk_recalc_rq_segments(&rq);
      bio->bi_next = nxt;
      bio->bi_phys_segments = rq.nr_phys_segments;
      bio->bi_hw_segments = rq.nr_hw_segments;
      bio->bi_flags |= (1 << BIO_SEG_VALID);
}
EXPORT_SYMBOL(blk_recount_segments);

static void blk_recalc_rq_segments(struct request *rq)
{
      int nr_phys_segs;
      int nr_hw_segs;
      unsigned int phys_size;
      unsigned int hw_size;
      struct bio_vec *bv, *bvprv = NULL;
      int seg_size;
      int hw_seg_size;
      int cluster;
      struct req_iterator iter;
      int high, highprv = 1;
      struct request_queue *q = rq->q;

      if (!rq->bio)
            return;

      cluster = q->queue_flags & (1 << QUEUE_FLAG_CLUSTER);
      hw_seg_size = seg_size = 0;
      phys_size = hw_size = nr_phys_segs = nr_hw_segs = 0;
      rq_for_each_segment(bv, rq, iter) {
            /*
             * the trick here is making sure that a high page is never
             * considered part of another segment, since that might
             * change with the bounce page.
             */
            high = page_to_pfn(bv->bv_page) > q->bounce_pfn;
            if (high || highprv)
                  goto new_hw_segment;
            if (cluster) {
                  if (seg_size + bv->bv_len > q->max_segment_size)
                        goto new_segment;
                  if (!BIOVEC_PHYS_MERGEABLE(bvprv, bv))
                        goto new_segment;
                  if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bv))
                        goto new_segment;
                  if (BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
                        goto new_hw_segment;

                  seg_size += bv->bv_len;
                  hw_seg_size += bv->bv_len;
                  bvprv = bv;
                  continue;
            }
new_segment:
            if (BIOVEC_VIRT_MERGEABLE(bvprv, bv) &&
                !BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
                  hw_seg_size += bv->bv_len;
            else {
new_hw_segment:
                  if (nr_hw_segs == 1 &&
                      hw_seg_size > rq->bio->bi_hw_front_size)
                        rq->bio->bi_hw_front_size = hw_seg_size;
                  hw_seg_size = BIOVEC_VIRT_START_SIZE(bv) + bv->bv_len;
                  nr_hw_segs++;
            }

            nr_phys_segs++;
            bvprv = bv;
            seg_size = bv->bv_len;
            highprv = high;
      }

      if (nr_hw_segs == 1 &&
          hw_seg_size > rq->bio->bi_hw_front_size)
            rq->bio->bi_hw_front_size = hw_seg_size;
      if (hw_seg_size > rq->biotail->bi_hw_back_size)
            rq->biotail->bi_hw_back_size = hw_seg_size;
      rq->nr_phys_segments = nr_phys_segs;
      rq->nr_hw_segments = nr_hw_segs;
}

static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
                           struct bio *nxt)
{
      if (!(q->queue_flags & (1 << QUEUE_FLAG_CLUSTER)))
            return 0;

      if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
            return 0;
      if (bio->bi_size + nxt->bi_size > q->max_segment_size)
            return 0;

      /*
       * bio and nxt are contigous in memory, check if the queue allows
       * these two to be merged into one
       */
      if (BIO_SEG_BOUNDARY(q, bio, nxt))
            return 1;

      return 0;
}

static int blk_hw_contig_segment(struct request_queue *q, struct bio *bio,
                         struct bio *nxt)
{
      if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
            blk_recount_segments(q, bio);
      if (unlikely(!bio_flagged(nxt, BIO_SEG_VALID)))
            blk_recount_segments(q, nxt);
      if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
          BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size))
            return 0;
      if (bio->bi_hw_back_size + nxt->bi_hw_front_size > q->max_segment_size)
            return 0;

      return 1;
}

/*
 * map a request to scatterlist, return number of sg entries setup. Caller
 * must make sure sg can hold rq->nr_phys_segments entries
 */
int blk_rq_map_sg(struct request_queue *q, struct request *rq,
              struct scatterlist *sglist)
{
      struct bio_vec *bvec, *bvprv;
      struct req_iterator iter;
      struct scatterlist *sg;
      int nsegs, cluster;

      nsegs = 0;
      cluster = q->queue_flags & (1 << QUEUE_FLAG_CLUSTER);

      /*
       * for each bio in rq
       */
      bvprv = NULL;
      sg = NULL;
      rq_for_each_segment(bvec, rq, iter) {
            int nbytes = bvec->bv_len;

            if (bvprv && cluster) {
                  if (sg->length + nbytes > q->max_segment_size)
                        goto new_segment;

                  if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
                        goto new_segment;
                  if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
                        goto new_segment;

                  sg->length += nbytes;
            } else {
new_segment:
                  if (!sg)
                        sg = sglist;
                  else {
                        /*
                         * If the driver previously mapped a shorter
                         * list, we could see a termination bit
                         * prematurely unless it fully inits the sg
                         * table on each mapping. We KNOW that there
                         * must be more entries here or the driver
                         * would be buggy, so force clear the
                         * termination bit to avoid doing a full
                         * sg_init_table() in drivers for each command.
                         */
                        sg->page_link &= ~0x02;
                        sg = sg_next(sg);
                  }

                  sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
                  nsegs++;
            }
            bvprv = bvec;
      } /* segments in rq */

      if (sg)
            sg_mark_end(sg);

      return nsegs;
}

EXPORT_SYMBOL(blk_rq_map_sg);

/*
 * the standard queue merge functions, can be overridden with device
 * specific ones if so desired
 */

static inline int ll_new_mergeable(struct request_queue *q,
                           struct request *req,
                           struct bio *bio)
{
      int nr_phys_segs = bio_phys_segments(q, bio);

      if (req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
            req->cmd_flags |= REQ_NOMERGE;
            if (req == q->last_merge)
                  q->last_merge = NULL;
            return 0;
      }

      /*
       * A hw segment is just getting larger, bump just the phys
       * counter.
       */
      req->nr_phys_segments += nr_phys_segs;
      return 1;
}

static inline int ll_new_hw_segment(struct request_queue *q,
                            struct request *req,
                            struct bio *bio)
{
      int nr_hw_segs = bio_hw_segments(q, bio);
      int nr_phys_segs = bio_phys_segments(q, bio);

      if (req->nr_hw_segments + nr_hw_segs > q->max_hw_segments
          || req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
            req->cmd_flags |= REQ_NOMERGE;
            if (req == q->last_merge)
                  q->last_merge = NULL;
            return 0;
      }

      /*
       * This will form the start of a new hw segment.  Bump both
       * counters.
       */
      req->nr_hw_segments += nr_hw_segs;
      req->nr_phys_segments += nr_phys_segs;
      return 1;
}

static int ll_back_merge_fn(struct request_queue *q, struct request *req,
                      struct bio *bio)
{
      unsigned short max_sectors;
      int len;

      if (unlikely(blk_pc_request(req)))
            max_sectors = q->max_hw_sectors;
      else
            max_sectors = q->max_sectors;

      if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
            req->cmd_flags |= REQ_NOMERGE;
            if (req == q->last_merge)
                  q->last_merge = NULL;
            return 0;
      }
      if (unlikely(!bio_flagged(req->biotail, BIO_SEG_VALID)))
            blk_recount_segments(q, req->biotail);
      if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
            blk_recount_segments(q, bio);
      len = req->biotail->bi_hw_back_size + bio->bi_hw_front_size;
      if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio)) &&
          !BIOVEC_VIRT_OVERSIZE(len)) {
            int mergeable =  ll_new_mergeable(q, req, bio);

            if (mergeable) {
                  if (req->nr_hw_segments == 1)
                        req->bio->bi_hw_front_size = len;
                  if (bio->bi_hw_segments == 1)
                        bio->bi_hw_back_size = len;
            }
            return mergeable;
      }

      return ll_new_hw_segment(q, req, bio);
}

static int ll_front_merge_fn(struct request_queue *q, struct request *req, 
                       struct bio *bio)
{
      unsigned short max_sectors;
      int len;

      if (unlikely(blk_pc_request(req)))
            max_sectors = q->max_hw_sectors;
      else
            max_sectors = q->max_sectors;


      if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
            req->cmd_flags |= REQ_NOMERGE;
            if (req == q->last_merge)
                  q->last_merge = NULL;
            return 0;
      }
      len = bio->bi_hw_back_size + req->bio->bi_hw_front_size;
      if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
            blk_recount_segments(q, bio);
      if (unlikely(!bio_flagged(req->bio, BIO_SEG_VALID)))
            blk_recount_segments(q, req->bio);
      if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
          !BIOVEC_VIRT_OVERSIZE(len)) {
            int mergeable =  ll_new_mergeable(q, req, bio);

            if (mergeable) {
                  if (bio->bi_hw_segments == 1)
                        bio->bi_hw_front_size = len;
                  if (req->nr_hw_segments == 1)
                        req->biotail->bi_hw_back_size = len;
            }
            return mergeable;
      }

      return ll_new_hw_segment(q, req, bio);
}

static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
                        struct request *next)
{
      int total_phys_segments;
      int total_hw_segments;

      /*
       * First check if the either of the requests are re-queued
       * requests.  Can't merge them if they are.
       */
      if (req->special || next->special)
            return 0;

      /*
       * Will it become too large?
       */
      if ((req->nr_sectors + next->nr_sectors) > q->max_sectors)
            return 0;

      total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
      if (blk_phys_contig_segment(q, req->biotail, next->bio))
            total_phys_segments--;

      if (total_phys_segments > q->max_phys_segments)
            return 0;

      total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
      if (blk_hw_contig_segment(q, req->biotail, next->bio)) {
            int len = req->biotail->bi_hw_back_size + next->bio->bi_hw_front_size;
            /*
             * propagate the combined length to the end of the requests
             */
            if (req->nr_hw_segments == 1)
                  req->bio->bi_hw_front_size = len;
            if (next->nr_hw_segments == 1)
                  next->biotail->bi_hw_back_size = len;
            total_hw_segments--;
      }

      if (total_hw_segments > q->max_hw_segments)
            return 0;

      /* Merge is OK... */
      req->nr_phys_segments = total_phys_segments;
      req->nr_hw_segments = total_hw_segments;
      return 1;
}

/*
 * "plug" the device if there are no outstanding requests: this will
 * force the transfer to start only after we have put all the requests
 * on the list.
 *
 * This is called with interrupts off and no requests on the queue and
 * with the queue lock held.
 */
void blk_plug_device(struct request_queue *q)
{
      WARN_ON(!irqs_disabled());

      /*
       * don't plug a stopped queue, it must be paired with blk_start_queue()
       * which will restart the queueing
       */
      if (blk_queue_stopped(q))
            return;

      if (!test_and_set_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags)) {
            mod_timer(&q->unplug_timer, jiffies + q->unplug_delay);
            blk_add_trace_generic(q, NULL, 0, BLK_TA_PLUG);
      }
}

EXPORT_SYMBOL(blk_plug_device);

/*
 * remove the queue from the plugged list, if present. called with
 * queue lock held and interrupts disabled.
 */
int blk_remove_plug(struct request_queue *q)
{
      WARN_ON(!irqs_disabled());

      if (!test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
            return 0;

      del_timer(&q->unplug_timer);
      return 1;
}

EXPORT_SYMBOL(blk_remove_plug);

/*
 * remove the plug and let it rip..
 */
void __generic_unplug_device(struct request_queue *q)
{
      if (unlikely(blk_queue_stopped(q)))
            return;

      if (!blk_remove_plug(q))
            return;

      q->request_fn(q);
}
EXPORT_SYMBOL(__generic_unplug_device);

/**
 * generic_unplug_device - fire a request queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   Linux uses plugging to build bigger requests queues before letting
 *   the device have at them. If a queue is plugged, the I/O scheduler
 *   is still adding and merging requests on the queue. Once the queue
 *   gets unplugged, the request_fn defined for the queue is invoked and
 *   transfers started.
 **/
void generic_unplug_device(struct request_queue *q)
{
      spin_lock_irq(q->queue_lock);
      __generic_unplug_device(q);
      spin_unlock_irq(q->queue_lock);
}
EXPORT_SYMBOL(generic_unplug_device);

static void blk_backing_dev_unplug(struct backing_dev_info *bdi,
                           struct page *page)
{
      struct request_queue *q = bdi->unplug_io_data;

      blk_unplug(q);
}

static void blk_unplug_work(struct work_struct *work)
{
      struct request_queue *q =
            container_of(work, struct request_queue, unplug_work);

      blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL,
                        q->rq.count[READ] + q->rq.count[WRITE]);

      q->unplug_fn(q);
}

static void blk_unplug_timeout(unsigned long data)
{
      struct request_queue *q = (struct request_queue *)data;

      blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_TIMER, NULL,
                        q->rq.count[READ] + q->rq.count[WRITE]);

      kblockd_schedule_work(&q->unplug_work);
}

void blk_unplug(struct request_queue *q)
{
      /*
       * devices don't necessarily have an ->unplug_fn defined
       */
      if (q->unplug_fn) {
            blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL,
                              q->rq.count[READ] + q->rq.count[WRITE]);

            q->unplug_fn(q);
      }
}
EXPORT_SYMBOL(blk_unplug);

/**
 * blk_start_queue - restart a previously stopped queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   blk_start_queue() will clear the stop flag on the queue, and call
 *   the request_fn for the queue if it was in a stopped state when
 *   entered. Also see blk_stop_queue(). Queue lock must be held.
 **/
void blk_start_queue(struct request_queue *q)
{
      WARN_ON(!irqs_disabled());

      clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);

      /*
       * one level of recursion is ok and is much faster than kicking
       * the unplug handling
       */
      if (!test_and_set_bit(QUEUE_FLAG_REENTER, &q->queue_flags)) {
            q->request_fn(q);
            clear_bit(QUEUE_FLAG_REENTER, &q->queue_flags);
      } else {
            blk_plug_device(q);
            kblockd_schedule_work(&q->unplug_work);
      }
}

EXPORT_SYMBOL(blk_start_queue);

/**
 * blk_stop_queue - stop a queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   The Linux block layer assumes that a block driver will consume all
 *   entries on the request queue when the request_fn strategy is called.
 *   Often this will not happen, because of hardware limitations (queue
 *   depth settings). If a device driver gets a 'queue full' response,
 *   or if it simply chooses not to queue more I/O at one point, it can
 *   call this function to prevent the request_fn from being called until
 *   the driver has signalled it's ready to go again. This happens by calling
 *   blk_start_queue() to restart queue operations. Queue lock must be held.
 **/
void blk_stop_queue(struct request_queue *q)
{
      blk_remove_plug(q);
      set_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
}
EXPORT_SYMBOL(blk_stop_queue);

/**
 * blk_sync_queue - cancel any pending callbacks on a queue
 * @q: the queue
 *
 * Description:
 *     The block layer may perform asynchronous callback activity
 *     on a queue, such as calling the unplug function after a timeout.
 *     A block device may call blk_sync_queue to ensure that any
 *     such activity is cancelled, thus allowing it to release resources
 *     that the callbacks might use. The caller must already have made sure
 *     that its ->make_request_fn will not re-add plugging prior to calling
 *     this function.
 *
 */
void blk_sync_queue(struct request_queue *q)
{
      del_timer_sync(&q->unplug_timer);
      kblockd_flush_work(&q->unplug_work);
}
EXPORT_SYMBOL(blk_sync_queue);

/**
 * blk_run_queue - run a single device queue
 * @q:      The queue to run
 */
void blk_run_queue(struct request_queue *q)
{
      unsigned long flags;

      spin_lock_irqsave(q->queue_lock, flags);
      blk_remove_plug(q);

      /*
       * Only recurse once to avoid overrunning the stack, let the unplug
       * handling reinvoke the handler shortly if we already got there.
       */
      if (!elv_queue_empty(q)) {
            if (!test_and_set_bit(QUEUE_FLAG_REENTER, &q->queue_flags)) {
                  q->request_fn(q);
                  clear_bit(QUEUE_FLAG_REENTER, &q->queue_flags);
            } else {
                  blk_plug_device(q);
                  kblockd_schedule_work(&q->unplug_work);
            }
      }

      spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_run_queue);

/**
 * blk_cleanup_queue: - release a &struct request_queue when it is no longer needed
 * @kobj:    the kobj belonging of the request queue to be released
 *
 * Description:
 *     blk_cleanup_queue is the pair to blk_init_queue() or
 *     blk_queue_make_request().  It should be called when a request queue is
 *     being released; typically when a block device is being de-registered.
 *     Currently, its primary task it to free all the &struct request
 *     structures that were allocated to the queue and the queue itself.
 *
 * Caveat:
 *     Hopefully the low level driver will have finished any
 *     outstanding requests first...
 **/
static void blk_release_queue(struct kobject *kobj)
{
      struct request_queue *q =
            container_of(kobj, struct request_queue, kobj);
      struct request_list *rl = &q->rq;

      blk_sync_queue(q);

      if (rl->rq_pool)
            mempool_destroy(rl->rq_pool);

      if (q->queue_tags)
            __blk_queue_free_tags(q);

      blk_trace_shutdown(q);

      bdi_destroy(&q->backing_dev_info);
      kmem_cache_free(requestq_cachep, q);
}

void blk_put_queue(struct request_queue *q)
{
      kobject_put(&q->kobj);
}
EXPORT_SYMBOL(blk_put_queue);

void blk_cleanup_queue(struct request_queue * q)
{
      mutex_lock(&q->sysfs_lock);
      set_bit(QUEUE_FLAG_DEAD, &q->queue_flags);
      mutex_unlock(&q->sysfs_lock);

      if (q->elevator)
            elevator_exit(q->elevator);

      blk_put_queue(q);
}

EXPORT_SYMBOL(blk_cleanup_queue);

static int blk_init_free_list(struct request_queue *q)
{
      struct request_list *rl = &q->rq;

      rl->count[READ] = rl->count[WRITE] = 0;
      rl->starved[READ] = rl->starved[WRITE] = 0;
      rl->elvpriv = 0;
      init_waitqueue_head(&rl->wait[READ]);
      init_waitqueue_head(&rl->wait[WRITE]);

      rl->rq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
                        mempool_free_slab, request_cachep, q->node);

      if (!rl->rq_pool)
            return -ENOMEM;

      return 0;
}

struct request_queue *blk_alloc_queue(gfp_t gfp_mask)
{
      return blk_alloc_queue_node(gfp_mask, -1);
}
EXPORT_SYMBOL(blk_alloc_queue);

static struct kobj_type queue_ktype;

struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
{
      struct request_queue *q;
      int err;

      q = kmem_cache_alloc_node(requestq_cachep,
                        gfp_mask | __GFP_ZERO, node_id);
      if (!q)
            return NULL;

      q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
      q->backing_dev_info.unplug_io_data = q;
      err = bdi_init(&q->backing_dev_info);
      if (err) {
            kmem_cache_free(requestq_cachep, q);
            return NULL;
      }

      init_timer(&q->unplug_timer);

      kobject_set_name(&q->kobj, "%s", "queue");
      q->kobj.ktype = &queue_ktype;
      kobject_init(&q->kobj);

      mutex_init(&q->sysfs_lock);

      return q;
}
EXPORT_SYMBOL(blk_alloc_queue_node);

/**
 * blk_init_queue  - prepare a request queue for use with a block device
 * @rfn:  The function to be called to process requests that have been
 *        placed on the queue.
 * @lock: Request queue spin lock
 *
 * Description:
 *    If a block device wishes to use the standard request handling procedures,
 *    which sorts requests and coalesces adjacent requests, then it must
 *    call blk_init_queue().  The function @rfn will be called when there
 *    are requests on the queue that need to be processed.  If the device
 *    supports plugging, then @rfn may not be called immediately when requests
 *    are available on the queue, but may be called at some time later instead.
 *    Plugged queues are generally unplugged when a buffer belonging to one
 *    of the requests on the queue is needed, or due to memory pressure.
 *
 *    @rfn is not required, or even expected, to remove all requests off the
 *    queue, but only as many as it can handle at a time.  If it does leave
 *    requests on the queue, it is responsible for arranging that the requests
 *    get dealt with eventually.
 *
 *    The queue spin lock must be held while manipulating the requests on the
 *    request queue; this lock will be taken also from interrupt context, so irq
 *    disabling is needed for it.
 *
 *    Function returns a pointer to the initialized request queue, or NULL if
 *    it didn't succeed.
 *
 * Note:
 *    blk_init_queue() must be paired with a blk_cleanup_queue() call
 *    when the block device is deactivated (such as at module unload).
 **/

struct request_queue *blk_init_queue(request_fn_proc *rfn, spinlock_t *lock)
{
      return blk_init_queue_node(rfn, lock, -1);
}
EXPORT_SYMBOL(blk_init_queue);

struct request_queue *
blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
{
      struct request_queue *q = blk_alloc_queue_node(GFP_KERNEL, node_id);

      if (!q)
            return NULL;

      q->node = node_id;
      if (blk_init_free_list(q)) {
            kmem_cache_free(requestq_cachep, q);
            return NULL;
      }

      /*
       * if caller didn't supply a lock, they get per-queue locking with
       * our embedded lock
       */
      if (!lock) {
            spin_lock_init(&q->__queue_lock);
            lock = &q->__queue_lock;
      }

      q->request_fn           = rfn;
      q->prep_rq_fn           = NULL;
      q->unplug_fn            = generic_unplug_device;
      q->queue_flags          = (1 << QUEUE_FLAG_CLUSTER);
      q->queue_lock           = lock;

      blk_queue_segment_boundary(q, 0xffffffff);

      blk_queue_make_request(q, __make_request);
      blk_queue_max_segment_size(q, MAX_SEGMENT_SIZE);

      blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
      blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);

      q->sg_reserved_size = INT_MAX;

      /*
       * all done
       */
      if (!elevator_init(q, NULL)) {
            blk_queue_congestion_threshold(q);
            return q;
      }

      blk_put_queue(q);
      return NULL;
}
EXPORT_SYMBOL(blk_init_queue_node);

int blk_get_queue(struct request_queue *q)
{
      if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
            kobject_get(&q->kobj);
            return 0;
      }

      return 1;
}

EXPORT_SYMBOL(blk_get_queue);

static inline void blk_free_request(struct request_queue *q, struct request *rq)
{
      if (rq->cmd_flags & REQ_ELVPRIV)
            elv_put_request(q, rq);
      mempool_free(rq, q->rq.rq_pool);
}

static struct request *
blk_alloc_request(struct request_queue *q, int rw, int priv, gfp_t gfp_mask)
{
      struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);

      if (!rq)
            return NULL;

      /*
       * first three bits are identical in rq->cmd_flags and bio->bi_rw,
       * see bio.h and blkdev.h
       */
      rq->cmd_flags = rw | REQ_ALLOCED;

      if (priv) {
            if (unlikely(elv_set_request(q, rq, gfp_mask))) {
                  mempool_free(rq, q->rq.rq_pool);
                  return NULL;
            }
            rq->cmd_flags |= REQ_ELVPRIV;
      }

      return rq;
}

/*
 * ioc_batching returns true if the ioc is a valid batching request and
 * should be given priority access to a request.
 */
static inline int ioc_batching(struct request_queue *q, struct io_context *ioc)
{
      if (!ioc)
            return 0;

      /*
       * Make sure the process is able to allocate at least 1 request
       * even if the batch times out, otherwise we could theoretically
       * lose wakeups.
       */
      return ioc->nr_batch_requests == q->nr_batching ||
            (ioc->nr_batch_requests > 0
            && time_before(jiffies, ioc->last_waited + BLK_BATCH_TIME));
}

/*
 * ioc_set_batching sets ioc to be a new "batcher" if it is not one. This
 * will cause the process to be a "batcher" on all queues in the system. This
 * is the behaviour we want though - once it gets a wakeup it should be given
 * a nice run.
 */
static void ioc_set_batching(struct request_queue *q, struct io_context *ioc)
{
      if (!ioc || ioc_batching(q, ioc))
            return;

      ioc->nr_batch_requests = q->nr_batching;
      ioc->last_waited = jiffies;
}

static void __freed_request(struct request_queue *q, int rw)
{
      struct request_list *rl = &q->rq;

      if (rl->count[rw] < queue_congestion_off_threshold(q))
            blk_clear_queue_congested(q, rw);

      if (rl->count[rw] + 1 <= q->nr_requests) {
            if (waitqueue_active(&rl->wait[rw]))
                  wake_up(&rl->wait[rw]);

            blk_clear_queue_full(q, rw);
      }
}

/*
 * A request has just been released.  Account for it, update the full and
 * congestion status, wake up any waiters.   Called under q->queue_lock.
 */
static void freed_request(struct request_queue *q, int rw, int priv)
{
      struct request_list *rl = &q->rq;

      rl->count[rw]--;
      if (priv)
            rl->elvpriv--;

      __freed_request(q, rw);

      if (unlikely(rl->starved[rw ^ 1]))
            __freed_request(q, rw ^ 1);
}

#define blkdev_free_rq(list) list_entry((list)->next, struct request, queuelist)
/*
 * Get a free request, queue_lock must be held.
 * Returns NULL on failure, with queue_lock held.
 * Returns !NULL on success, with queue_lock *not held*.
 */
static struct request *get_request(struct request_queue *q, int rw_flags,
                           struct bio *bio, gfp_t gfp_mask)
{
      struct request *rq = NULL;
      struct request_list *rl = &q->rq;
      struct io_context *ioc = NULL;
      const int rw = rw_flags & 0x01;
      int may_queue, priv;

      may_queue = elv_may_queue(q, rw_flags);
      if (may_queue == ELV_MQUEUE_NO)
            goto rq_starved;

      if (rl->count[rw]+1 >= queue_congestion_on_threshold(q)) {
            if (rl->count[rw]+1 >= q->nr_requests) {
                  ioc = current_io_context(GFP_ATOMIC, q->node);
                  /*
                   * The queue will fill after this allocation, so set
                   * it as full, and mark this process as "batching".
                   * This process will be allowed to complete a batch of
                   * requests, others will be blocked.
                   */
                  if (!blk_queue_full(q, rw)) {
                        ioc_set_batching(q, ioc);
                        blk_set_queue_full(q, rw);
                  } else {
                        if (may_queue != ELV_MQUEUE_MUST
                                    && !ioc_batching(q, ioc)) {
                              /*
                               * The queue is full and the allocating
                               * process is not a "batcher", and not
                               * exempted by the IO scheduler
                               */
                              goto out;
                        }
                  }
            }
            blk_set_queue_congested(q, rw);
      }

      /*
       * Only allow batching queuers to allocate up to 50% over the defined
       * limit of requests, otherwise we could have thousands of requests
       * allocated with any setting of ->nr_requests
       */
      if (rl->count[rw] >= (3 * q->nr_requests / 2))
            goto out;

      rl->count[rw]++;
      rl->starved[rw] = 0;

      priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
      if (priv)
            rl->elvpriv++;

      spin_unlock_irq(q->queue_lock);

      rq = blk_alloc_request(q, rw_flags, priv, gfp_mask);
      if (unlikely(!rq)) {
            /*
             * Allocation failed presumably due to memory. Undo anything
             * we might have messed up.
             *
             * Allocating task should really be put onto the front of the
             * wait queue, but this is pretty rare.
             */
            spin_lock_irq(q->queue_lock);
            freed_request(q, rw, priv);

            /*
             * in the very unlikely event that allocation failed and no
             * requests for this direction was pending, mark us starved
             * so that freeing of a request in the other direction will
             * notice us. another possible fix would be to split the
             * rq mempool into READ and WRITE
             */
rq_starved:
            if (unlikely(rl->count[rw] == 0))
                  rl->starved[rw] = 1;

            goto out;
      }

      /*
       * ioc may be NULL here, and ioc_batching will be false. That's
       * OK, if the queue is under the request limit then requests need
       * not count toward the nr_batch_requests limit. There will always
       * be some limit enforced by BLK_BATCH_TIME.
       */
      if (ioc_batching(q, ioc))
            ioc->nr_batch_requests--;
      
      rq_init(q, rq);

      blk_add_trace_generic(q, bio, rw, BLK_TA_GETRQ);
out:
      return rq;
}

/*
 * No available requests for this queue, unplug the device and wait for some
 * requests to become available.
 *
 * Called with q->queue_lock held, and returns with it unlocked.
 */
static struct request *get_request_wait(struct request_queue *q, int rw_flags,
                              struct bio *bio)
{
      const int rw = rw_flags & 0x01;
      struct request *rq;

      rq = get_request(q, rw_flags, bio, GFP_NOIO);
      while (!rq) {
            DEFINE_WAIT(wait);
            struct request_list *rl = &q->rq;

            prepare_to_wait_exclusive(&rl->wait[rw], &wait,
                        TASK_UNINTERRUPTIBLE);

            rq = get_request(q, rw_flags, bio, GFP_NOIO);

            if (!rq) {
                  struct io_context *ioc;

                  blk_add_trace_generic(q, bio, rw, BLK_TA_SLEEPRQ);

                  __generic_unplug_device(q);
                  spin_unlock_irq(q->queue_lock);
                  io_schedule();

                  /*
                   * After sleeping, we become a "batching" process and
                   * will be able to allocate at least one request, and
                   * up to a big batch of them for a small period time.
                   * See ioc_batching, ioc_set_batching
                   */
                  ioc = current_io_context(GFP_NOIO, q->node);
                  ioc_set_batching(q, ioc);

                  spin_lock_irq(q->queue_lock);
            }
            finish_wait(&rl->wait[rw], &wait);
      }

      return rq;
}

struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
{
      struct request *rq;

      BUG_ON(rw != READ && rw != WRITE);

      spin_lock_irq(q->queue_lock);
      if (gfp_mask & __GFP_WAIT) {
            rq = get_request_wait(q, rw, NULL);
      } else {
            rq = get_request(q, rw, NULL, gfp_mask);
            if (!rq)
                  spin_unlock_irq(q->queue_lock);
      }
      /* q->queue_lock is unlocked at this point */

      return rq;
}
EXPORT_SYMBOL(blk_get_request);

/**
 * blk_start_queueing - initiate dispatch of requests to device
 * @q:            request queue to kick into gear
 *
 * This is basically a helper to remove the need to know whether a queue
 * is plugged or not if someone just wants to initiate dispatch of requests
 * for this queue.
 *
 * The queue lock must be held with interrupts disabled.
 */
void blk_start_queueing(struct request_queue *q)
{
      if (!blk_queue_plugged(q))
            q->request_fn(q);
      else
            __generic_unplug_device(q);
}
EXPORT_SYMBOL(blk_start_queueing);

/**
 * blk_requeue_request - put a request back on queue
 * @q:            request queue where request should be inserted
 * @rq:           request to be inserted
 *
 * Description:
 *    Drivers often keep queueing requests until the hardware cannot accept
 *    more, when that condition happens we need to put the request back
 *    on the queue. Must be called with queue lock held.
 */
void blk_requeue_request(struct request_queue *q, struct request *rq)
{
      blk_add_trace_rq(q, rq, BLK_TA_REQUEUE);

      if (blk_rq_tagged(rq))
            blk_queue_end_tag(q, rq);

      elv_requeue_request(q, rq);
}

EXPORT_SYMBOL(blk_requeue_request);

/**
 * blk_insert_request - insert a special request in to a request queue
 * @q:            request queue where request should be inserted
 * @rq:           request to be inserted
 * @at_head:      insert request at head or tail of queue
 * @data:   private data
 *
 * Description:
 *    Many block devices need to execute commands asynchronously, so they don't
 *    block the whole kernel from preemption during request execution.  This is
 *    accomplished normally by inserting aritficial requests tagged as
 *    REQ_SPECIAL in to the corresponding request queue, and letting them be
 *    scheduled for actual execution by the request queue.
 *
 *    We have the option of inserting the head or the tail of the queue.
 *    Typically we use the tail for new ioctls and so forth.  We use the head
 *    of the queue for things like a QUEUE_FULL message from a device, or a
 *    host that is unable to accept a particular command.
 */
void blk_insert_request(struct request_queue *q, struct request *rq,
                  int at_head, void *data)
{
      int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
      unsigned long flags;

      /*
       * tell I/O scheduler that this isn't a regular read/write (ie it
       * must not attempt merges on this) and that it acts as a soft
       * barrier
       */
      rq->cmd_type = REQ_TYPE_SPECIAL;
      rq->cmd_flags |= REQ_SOFTBARRIER;

      rq->special = data;

      spin_lock_irqsave(q->queue_lock, flags);

      /*
       * If command is tagged, release the tag
       */
      if (blk_rq_tagged(rq))
            blk_queue_end_tag(q, rq);

      drive_stat_acct(rq, 1);
      __elv_add_request(q, rq, where, 0);
      blk_start_queueing(q);
      spin_unlock_irqrestore(q->queue_lock, flags);
}

EXPORT_SYMBOL(blk_insert_request);

static int __blk_rq_unmap_user(struct bio *bio)
{
      int ret = 0;

      if (bio) {
            if (bio_flagged(bio, BIO_USER_MAPPED))
                  bio_unmap_user(bio);
            else
                  ret = bio_uncopy_user(bio);
      }

      return ret;
}

int blk_rq_append_bio(struct request_queue *q, struct request *rq,
                  struct bio *bio)
{
      if (!rq->bio)
            blk_rq_bio_prep(q, rq, bio);
      else if (!ll_back_merge_fn(q, rq, bio))
            return -EINVAL;
      else {
            rq->biotail->bi_next = bio;
            rq->biotail = bio;

            rq->data_len += bio->bi_size;
      }
      return 0;
}
EXPORT_SYMBOL(blk_rq_append_bio);

static int __blk_rq_map_user(struct request_queue *q, struct request *rq,
                       void __user *ubuf, unsigned int len)
{
      unsigned long uaddr;
      struct bio *bio, *orig_bio;
      int reading, ret;

      reading = rq_data_dir(rq) == READ;

      /*
       * if alignment requirement is satisfied, map in user pages for
       * direct dma. else, set up kernel bounce buffers
       */
      uaddr = (unsigned long) ubuf;
      if (!(uaddr & queue_dma_alignment(q)) && !(len & queue_dma_alignment(q)))
            bio = bio_map_user(q, NULL, uaddr, len, reading);
      else
            bio = bio_copy_user(q, uaddr, len, reading);

      if (IS_ERR(bio))
            return PTR_ERR(bio);

      orig_bio = bio;
      blk_queue_bounce(q, &bio);

      /*
       * We link the bounce buffer in and could have to traverse it
       * later so we have to get a ref to prevent it from being freed
       */
      bio_get(bio);

      ret = blk_rq_append_bio(q, rq, bio);
      if (!ret)
            return bio->bi_size;

      /* if it was boucned we must call the end io function */
      bio_endio(bio, 0);
      __blk_rq_unmap_user(orig_bio);
      bio_put(bio);
      return ret;
}

/**
 * blk_rq_map_user - map user data to a request, for REQ_BLOCK_PC usage
 * @q:            request queue where request should be inserted
 * @rq:           request structure to fill
 * @ubuf:   the user buffer
 * @len:    length of user data
 *
 * Description:
 *    Data will be mapped directly for zero copy io, if possible. Otherwise
 *    a kernel bounce buffer is used.
 *
 *    A matching blk_rq_unmap_user() must be issued at the end of io, while
 *    still in process context.
 *
 *    Note: The mapped bio may need to be bounced through blk_queue_bounce()
 *    before being submitted to the device, as pages mapped may be out of
 *    reach. It's the callers responsibility to make sure this happens. The
 *    original bio must be passed back in to blk_rq_unmap_user() for proper
 *    unmapping.
 */
int blk_rq_map_user(struct request_queue *q, struct request *rq,
                void __user *ubuf, unsigned long len)
{
      unsigned long bytes_read = 0;
      struct bio *bio = NULL;
      int ret;

      if (len > (q->max_hw_sectors << 9))
            return -EINVAL;
      if (!len || !ubuf)
            return -EINVAL;

      while (bytes_read != len) {
            unsigned long map_len, end, start;

            map_len = min_t(unsigned long, len - bytes_read, BIO_MAX_SIZE);
            end = ((unsigned long)ubuf + map_len + PAGE_SIZE - 1)
                                                >> PAGE_SHIFT;
            start = (unsigned long)ubuf >> PAGE_SHIFT;

            /*
             * A bad offset could cause us to require BIO_MAX_PAGES + 1
             * pages. If this happens we just lower the requested
             * mapping len by a page so that we can fit
             */
            if (end - start > BIO_MAX_PAGES)
                  map_len -= PAGE_SIZE;

            ret = __blk_rq_map_user(q, rq, ubuf, map_len);
            if (ret < 0)
                  goto unmap_rq;
            if (!bio)
                  bio = rq->bio;
            bytes_read += ret;
            ubuf += ret;
      }

      rq->buffer = rq->data = NULL;
      return 0;
unmap_rq:
      blk_rq_unmap_user(bio);
      return ret;
}

EXPORT_SYMBOL(blk_rq_map_user);

/**
 * blk_rq_map_user_iov - map user data to a request, for REQ_BLOCK_PC usage
 * @q:            request queue where request should be inserted
 * @rq:           request to map data to
 * @iov:    pointer to the iovec
 * @iov_count:    number of elements in the iovec
 * @len:    I/O byte count
 *
 * Description:
 *    Data will be mapped directly for zero copy io, if possible. Otherwise
 *    a kernel bounce buffer is used.
 *
 *    A matching blk_rq_unmap_user() must be issued at the end of io, while
 *    still in process context.
 *
 *    Note: The mapped bio may need to be bounced through blk_queue_bounce()
 *    before being submitted to the device, as pages mapped may be out of
 *    reach. It's the callers responsibility to make sure this happens. The
 *    original bio must be passed back in to blk_rq_unmap_user() for proper
 *    unmapping.
 */
int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
                  struct sg_iovec *iov, int iov_count, unsigned int len)
{
      struct bio *bio;

      if (!iov || iov_count <= 0)
            return -EINVAL;

      /* we don't allow misaligned data like bio_map_user() does.  If the
       * user is using sg, they're expected to know the alignment constraints
       * and respect them accordingly */
      bio = bio_map_user_iov(q, NULL, iov, iov_count, rq_data_dir(rq)== READ);
      if (IS_ERR(bio))
            return PTR_ERR(bio);

      if (bio->bi_size != len) {
            bio_endio(bio, 0);
            bio_unmap_user(bio);
            return -EINVAL;
      }

      bio_get(bio);
      blk_rq_bio_prep(q, rq, bio);
      rq->buffer = rq->data = NULL;
      return 0;
}

EXPORT_SYMBOL(blk_rq_map_user_iov);

/**
 * blk_rq_unmap_user - unmap a request with user data
 * @bio:           start of bio list
 *
 * Description:
 *    Unmap a rq previously mapped by blk_rq_map_user(). The caller must
 *    supply the original rq->bio from the blk_rq_map_user() return, since
 *    the io completion may have changed rq->bio.
 */
int blk_rq_unmap_user(struct bio *bio)
{
      struct bio *mapped_bio;
      int ret = 0, ret2;

      while (bio) {
            mapped_bio = bio;
            if (unlikely(bio_flagged(bio, BIO_BOUNCED)))
                  mapped_bio = bio->bi_private;

            ret2 = __blk_rq_unmap_user(mapped_bio);
            if (ret2 && !ret)
                  ret = ret2;

            mapped_bio = bio;
            bio = bio->bi_next;
            bio_put(mapped_bio);
      }

      return ret;
}

EXPORT_SYMBOL(blk_rq_unmap_user);

/**
 * blk_rq_map_kern - map kernel data to a request, for REQ_BLOCK_PC usage
 * @q:            request queue where request should be inserted
 * @rq:           request to fill
 * @kbuf:   the kernel buffer
 * @len:    length of user data
 * @gfp_mask:     memory allocation flags
 */
int blk_rq_map_kern(struct request_queue *q, struct request *rq, void *kbuf,
                unsigned int len, gfp_t gfp_mask)
{
      struct bio *bio;

      if (len > (q->max_hw_sectors << 9))
            return -EINVAL;
      if (!len || !kbuf)
            return -EINVAL;

      bio = bio_map_kern(q, kbuf, len, gfp_mask);
      if (IS_ERR(bio))
            return PTR_ERR(bio);

      if (rq_data_dir(rq) == WRITE)
            bio->bi_rw |= (1 << BIO_RW);

      blk_rq_bio_prep(q, rq, bio);
      blk_queue_bounce(q, &rq->bio);
      rq->buffer = rq->data = NULL;
      return 0;
}

EXPORT_SYMBOL(blk_rq_map_kern);

/**
 * blk_execute_rq_nowait - insert a request into queue for execution
 * @q:            queue to insert the request in
 * @bd_disk:      matching gendisk
 * @rq:           request to insert
 * @at_head:    insert request at head or tail of queue
 * @done:   I/O completion handler
 *
 * Description:
 *    Insert a fully prepared request at the back of the io scheduler queue
 *    for execution.  Don't wait for completion.
 */
void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
                     struct request *rq, int at_head,
                     rq_end_io_fn *done)
{
      int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;

      rq->rq_disk = bd_disk;
      rq->cmd_flags |= REQ_NOMERGE;
      rq->end_io = done;
      WARN_ON(irqs_disabled());
      spin_lock_irq(q->queue_lock);
      __elv_add_request(q, rq, where, 1);
      __generic_unplug_device(q);
      spin_unlock_irq(q->queue_lock);
}
EXPORT_SYMBOL_GPL(blk_execute_rq_nowait);

/**
 * blk_execute_rq - insert a request into queue for execution
 * @q:            queue to insert the request in
 * @bd_disk:      matching gendisk
 * @rq:           request to insert
 * @at_head:    insert request at head or tail of queue
 *
 * Description:
 *    Insert a fully prepared request at the back of the io scheduler queue
 *    for execution and wait for completion.
 */
int blk_execute_rq(struct request_queue *q, struct gendisk *bd_disk,
               struct request *rq, int at_head)
{
      DECLARE_COMPLETION_ONSTACK(wait);
      char sense[SCSI_SENSE_BUFFERSIZE];
      int err = 0;

      /*
       * we need an extra reference to the request, so we can look at
       * it after io completion
       */
      rq->ref_count++;

      if (!rq->sense) {
            memset(sense, 0, sizeof(sense));
            rq->sense = sense;
            rq->sense_len = 0;
      }

      rq->end_io_data = &wait;
      blk_execute_rq_nowait(q, bd_disk, rq, at_head, blk_end_sync_rq);
      wait_for_completion(&wait);

      if (rq->errors)
            err = -EIO;

      return err;
}

EXPORT_SYMBOL(blk_execute_rq);

static void bio_end_empty_barrier(struct bio *bio, int err)
{
      if (err)
            clear_bit(BIO_UPTODATE, &bio->bi_flags);

      complete(bio->bi_private);
}

/**
 * blkdev_issue_flush - queue a flush
 * @bdev:   blockdev to issue flush for
 * @error_sector: error sector
 *
 * Description:
 *    Issue a flush for the block device in question. Caller can supply
 *    room for storing the error offset in case of a flush error, if they
 *    wish to.  Caller must run wait_for_completion() on its own.
 */
int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
{
      DECLARE_COMPLETION_ONSTACK(wait);
      struct request_queue *q;
      struct bio *bio;
      int ret;

      if (bdev->bd_disk == NULL)
            return -ENXIO;

      q = bdev_get_queue(bdev);
      if (!q)
            return -ENXIO;

      bio = bio_alloc(GFP_KERNEL, 0);
      if (!bio)
            return -ENOMEM;

      bio->bi_end_io = bio_end_empty_barrier;
      bio->bi_private = &wait;
      bio->bi_bdev = bdev;
      submit_bio(1 << BIO_RW_BARRIER, bio);

      wait_for_completion(&wait);

      /*
       * The driver must store the error location in ->bi_sector, if
       * it supports it. For non-stacked drivers, this should be copied
       * from rq->sector.
       */
      if (error_sector)
            *error_sector = bio->bi_sector;

      ret = 0;
      if (!bio_flagged(bio, BIO_UPTODATE))
            ret = -EIO;

      bio_put(bio);
      return ret;
}

EXPORT_SYMBOL(blkdev_issue_flush);

static void drive_stat_acct(struct request *rq, int new_io)
{
      int rw = rq_data_dir(rq);

      if (!blk_fs_request(rq) || !rq->rq_disk)
            return;

      if (!new_io) {
            __disk_stat_inc(rq->rq_disk, merges[rw]);
      } else {
            disk_round_stats(rq->rq_disk);
            rq->rq_disk->in_flight++;
      }
}

/*
 * add-request adds a request to the linked list.
 * queue lock is held and interrupts disabled, as we muck with the
 * request queue list.
 */
static inline void add_request(struct request_queue * q, struct request * req)
{
      drive_stat_acct(req, 1);

      /*
       * elevator indicated where it wants this request to be
       * inserted at elevator_merge time
       */
      __elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
}
 
/*
 * disk_round_stats()   - Round off the performance stats on a struct
 * disk_stats.
 *
 * The average IO queue length and utilisation statistics are maintained
 * by observing the current state of the queue length and the amount of
 * time it has been in this state for.
 *
 * Normally, that accounting is done on IO completion, but that can result
 * in more than a second's worth of IO being accounted for within any one
 * second, leading to >100% utilisation.  To deal with that, we call this
 * function to do a round-off before returning the results when reading
 * /proc/diskstats.  This accounts immediately for all queue usage up to
 * the current jiffies and restarts the counters again.
 */
void disk_round_stats(struct gendisk *disk)
{
      unsigned long now = jiffies;

      if (now == disk->stamp)
            return;

      if (disk->in_flight) {
            __disk_stat_add(disk, time_in_queue,
                        disk->in_flight * (now - disk->stamp));
            __disk_stat_add(disk, io_ticks, (now - disk->stamp));
      }
      disk->stamp = now;
}

EXPORT_SYMBOL_GPL(disk_round_stats);

/*
 * queue lock must be held
 */
void __blk_put_request(struct request_queue *q, struct request *req)
{
      if (unlikely(!q))
            return;
      if (unlikely(--req->ref_count))
            return;

      elv_completed_request(q, req);

      /*
       * Request may not have originated from ll_rw_blk. if not,
       * it didn't come out of our reserved rq pools
       */
      if (req->cmd_flags & REQ_ALLOCED) {
            int rw = rq_data_dir(req);
            int priv = req->cmd_flags & REQ_ELVPRIV;

            BUG_ON(!list_empty(&req->queuelist));
            BUG_ON(!hlist_unhashed(&req->hash));

            blk_free_request(q, req);
            freed_request(q, rw, priv);
      }
}

EXPORT_SYMBOL_GPL(__blk_put_request);

void blk_put_request(struct request *req)
{
      unsigned long flags;
      struct request_queue *q = req->q;

      /*
       * Gee, IDE calls in w/ NULL q.  Fix IDE and remove the
       * following if (q) test.
       */
      if (q) {
            spin_lock_irqsave(q->queue_lock, flags);
            __blk_put_request(q, req);
            spin_unlock_irqrestore(q->queue_lock, flags);
      }
}

EXPORT_SYMBOL(blk_put_request);

/**
 * blk_end_sync_rq - executes a completion event on a request
 * @rq: request to complete
 * @error: end io status of the request
 */
void blk_end_sync_rq(struct request *rq, int error)
{
      struct completion *waiting = rq->end_io_data;

      rq->end_io_data = NULL;
      __blk_put_request(rq->q, rq);

      /*
       * complete last, if this is a stack request the process (and thus
       * the rq pointer) could be invalid right after this complete()
       */
      complete(waiting);
}
EXPORT_SYMBOL(blk_end_sync_rq);

/*
 * Has to be called with the request spinlock acquired
 */
static int attempt_merge(struct request_queue *q, struct request *req,
                    struct request *next)
{
      if (!rq_mergeable(req) || !rq_mergeable(next))
            return 0;

      /*
       * not contiguous
       */
      if (req->sector + req->nr_sectors != next->sector)
            return 0;

      if (rq_data_dir(req) != rq_data_dir(next)
          || req->rq_disk != next->rq_disk
          || next->special)
            return 0;

      /*
       * If we are allowed to merge, then append bio list
       * from next to rq and release next. merge_requests_fn
       * will have updated segment counts, update sector
       * counts here.
       */
      if (!ll_merge_requests_fn(q, req, next))
            return 0;

      /*
       * At this point we have either done a back merge
       * or front merge. We need the smaller start_time of
       * the merged requests to be the current request
       * for accounting purposes.
       */
      if (time_after(req->start_time, next->start_time))
            req->start_time = next->start_time;

      req->biotail->bi_next = next->bio;
      req->biotail = next->biotail;

      req->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;

      elv_merge_requests(q, req, next);

      if (req->rq_disk) {
            disk_round_stats(req->rq_disk);
            req->rq_disk->in_flight--;
      }

      req->ioprio = ioprio_best(req->ioprio, next->ioprio);

      __blk_put_request(q, next);
      return 1;
}

static inline int attempt_back_merge(struct request_queue *q,
                             struct request *rq)
{
      struct request *next = elv_latter_request(q, rq);

      if (next)
            return attempt_merge(q, rq, next);

      return 0;
}

static inline int attempt_front_merge(struct request_queue *q,
                              struct request *rq)
{
      struct request *prev = elv_former_request(q, rq);

      if (prev)
            return attempt_merge(q, prev, rq);

      return 0;
}

static void init_request_from_bio(struct request *req, struct bio *bio)
{
      req->cmd_type = REQ_TYPE_FS;

      /*
       * inherit FAILFAST from bio (for read-ahead, and explicit FAILFAST)
       */
      if (bio_rw_ahead(bio) || bio_failfast(bio))
            req->cmd_flags |= REQ_FAILFAST;

      /*
       * REQ_BARRIER implies no merging, but lets make it explicit
       */
      if (unlikely(bio_barrier(bio)))
            req->cmd_flags |= (REQ_HARDBARRIER | REQ_NOMERGE);

      if (bio_sync(bio))
            req->cmd_flags |= REQ_RW_SYNC;
      if (bio_rw_meta(bio))
            req->cmd_flags |= REQ_RW_META;

      req->errors = 0;
      req->hard_sector = req->sector = bio->bi_sector;
      req->ioprio = bio_prio(bio);
      req->start_time = jiffies;
      blk_rq_bio_prep(req->q, req, bio);
}

static int __make_request(struct request_queue *q, struct bio *bio)
{
      struct request *req;
      int el_ret, nr_sectors, barrier, err;
      const unsigned short prio = bio_prio(bio);
      const int sync = bio_sync(bio);
      int rw_flags;

      nr_sectors = bio_sectors(bio);

      /*
       * low level driver can indicate that it wants pages above a
       * certain limit bounced to low memory (ie for highmem, or even
       * ISA dma in theory)
       */
      blk_queue_bounce(q, &bio);

      barrier = bio_barrier(bio);
      if (unlikely(barrier) && (q->next_ordered == QUEUE_ORDERED_NONE)) {
            err = -EOPNOTSUPP;
            goto end_io;
      }

      spin_lock_irq(q->queue_lock);

      if (unlikely(barrier) || elv_queue_empty(q))
            goto get_rq;

      el_ret = elv_merge(q, &req, bio);
      switch (el_ret) {
            case ELEVATOR_BACK_MERGE:
                  BUG_ON(!rq_mergeable(req));

                  if (!ll_back_merge_fn(q, req, bio))
                        break;

                  blk_add_trace_bio(q, bio, BLK_TA_BACKMERGE);

                  req->biotail->bi_next = bio;
                  req->biotail = bio;
                  req->nr_sectors = req->hard_nr_sectors += nr_sectors;
                  req->ioprio = ioprio_best(req->ioprio, prio);
                  drive_stat_acct(req, 0);
                  if (!attempt_back_merge(q, req))
                        elv_merged_request(q, req, el_ret);
                  goto out;

            case ELEVATOR_FRONT_MERGE:
                  BUG_ON(!rq_mergeable(req));

                  if (!ll_front_merge_fn(q, req, bio))
                        break;

                  blk_add_trace_bio(q, bio, BLK_TA_FRONTMERGE);

                  bio->bi_next = req->bio;
                  req->bio = bio;

                  /*
                   * may not be valid. if the low level driver said
                   * it didn't need a bounce buffer then it better
                   * not touch req->buffer either...
                   */
                  req->buffer = bio_data(bio);
                  req->current_nr_sectors = bio_cur_sectors(bio);
                  req->hard_cur_sectors = req->current_nr_sectors;
                  req->sector = req->hard_sector = bio->bi_sector;
                  req->nr_sectors = req->hard_nr_sectors += nr_sectors;
                  req->ioprio = ioprio_best(req->ioprio, prio);
                  drive_stat_acct(req, 0);
                  if (!attempt_front_merge(q, req))
                        elv_merged_request(q, req, el_ret);
                  goto out;

            /* ELV_NO_MERGE: elevator says don't/can't merge. */
            default:
                  ;
      }

get_rq:
      /*
       * This sync check and mask will be re-done in init_request_from_bio(),
       * but we need to set it earlier to expose the sync flag to the
       * rq allocator and io schedulers.
       */
      rw_flags = bio_data_dir(bio);
      if (sync)
            rw_flags |= REQ_RW_SYNC;

      /*
       * Grab a free request. This is might sleep but can not fail.
       * Returns with the queue unlocked.
       */
      req = get_request_wait(q, rw_flags, bio);

      /*
       * After dropping the lock and possibly sleeping here, our request
       * may now be mergeable after it had proven unmergeable (above).
       * We don't worry about that case for efficiency. It won't happen
       * often, and the elevators are able to handle it.
       */
      init_request_from_bio(req, bio);

      spin_lock_irq(q->queue_lock);
      if (elv_queue_empty(q))
            blk_plug_device(q);
      add_request(q, req);
out:
      if (sync)
            __generic_unplug_device(q);

      spin_unlock_irq(q->queue_lock);
      return 0;

end_io:
      bio_endio(bio, err);
      return 0;
}

/*
 * If bio->bi_dev is a partition, remap the location
 */
static inline void blk_partition_remap(struct bio *bio)
{
      struct block_device *bdev = bio->bi_bdev;

      if (bio_sectors(bio) && bdev != bdev->bd_contains) {
            struct hd_struct *p = bdev->bd_part;
            const int rw = bio_data_dir(bio);

            p->sectors[rw] += bio_sectors(bio);
            p->ios[rw]++;

            bio->bi_sector += p->start_sect;
            bio->bi_bdev = bdev->bd_contains;

            blk_add_trace_remap(bdev_get_queue(bio->bi_bdev), bio,
                            bdev->bd_dev, bio->bi_sector,
                            bio->bi_sector - p->start_sect);
      }
}

static void handle_bad_sector(struct bio *bio)
{
      char b[BDEVNAME_SIZE];

      printk(KERN_INFO "attempt to access beyond end of device\n");
      printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu\n",
                  bdevname(bio->bi_bdev, b),
                  bio->bi_rw,
                  (unsigned long long)bio->bi_sector + bio_sectors(bio),
                  (long long)(bio->bi_bdev->bd_inode->i_size >> 9));

      set_bit(BIO_EOF, &bio->bi_flags);
}

#ifdef CONFIG_FAIL_MAKE_REQUEST

static DECLARE_FAULT_ATTR(fail_make_request);

static int __init setup_fail_make_request(char *str)
{
      return setup_fault_attr(&fail_make_request, str);
}
__setup("fail_make_request=", setup_fail_make_request);

static int should_fail_request(struct bio *bio)
{
      if ((bio->bi_bdev->bd_disk->flags & GENHD_FL_FAIL) ||
          (bio->bi_bdev->bd_part && bio->bi_bdev->bd_part->make_it_fail))
            return should_fail(&fail_make_request, bio->bi_size);

      return 0;
}

static int __init fail_make_request_debugfs(void)
{
      return init_fault_attr_dentries(&fail_make_request,
                              "fail_make_request");
}

late_initcall(fail_make_request_debugfs);

#else /* CONFIG_FAIL_MAKE_REQUEST */

static inline int should_fail_request(struct bio *bio)
{
      return 0;
}

#endif /* CONFIG_FAIL_MAKE_REQUEST */

/*
 * Check whether this bio extends beyond the end of the device.
 */
static inline int bio_check_eod(struct bio *bio, unsigned int nr_sectors)
{
      sector_t maxsector;

      if (!nr_sectors)
            return 0;

      /* Test device or partition size, when known. */
      maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
      if (maxsector) {
            sector_t sector = bio->bi_sector;

            if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
                  /*
                   * This may well happen - the kernel calls bread()
                   * without checking the size of the device, e.g., when
                   * mounting a device.
                   */
                  handle_bad_sector(bio);
                  return 1;
            }
      }

      return 0;
}

/**
 * generic_make_request: hand a buffer to its device driver for I/O
 * @bio:  The bio describing the location in memory and on the device.
 *
 * generic_make_request() is used to make I/O requests of block
 * devices. It is passed a &struct bio, which describes the I/O that needs
 * to be done.
 *
 * generic_make_request() does not return any status.  The
 * success/failure status of the request, along with notification of
 * completion, is delivered asynchronously through the bio->bi_end_io
 * function described (one day) else where.
 *
 * The caller of generic_make_request must make sure that bi_io_vec
 * are set to describe the memory buffer, and that bi_dev and bi_sector are
 * set to describe the device address, and the
 * bi_end_io and optionally bi_private are set to describe how
 * completion notification should be signaled.
 *
 * generic_make_request and the drivers it calls may use bi_next if this
 * bio happens to be merged with someone else, and may change bi_dev and
 * bi_sector for remaps as it sees fit.  So the values of these fields
 * should NOT be depended on after the call to generic_make_request.
 */
static inline void __generic_make_request(struct bio *bio)
{
      struct request_queue *q;
      sector_t old_sector;
      int ret, nr_sectors = bio_sectors(bio);
      dev_t old_dev;
      int err = -EIO;

      might_sleep();

      if (bio_check_eod(bio, nr_sectors))
            goto end_io;

      /*
       * Resolve the mapping until finished. (drivers are
       * still free to implement/resolve their own stacking
       * by explicitly returning 0)
       *
       * NOTE: we don't repeat the blk_size check for each new device.
       * Stacking drivers are expected to know what they are doing.
       */
      old_sector = -1;
      old_dev = 0;
      do {
            char b[BDEVNAME_SIZE];

            q = bdev_get_queue(bio->bi_bdev);
            if (!q) {
                  printk(KERN_ERR
                         "generic_make_request: Trying to access "
                        "nonexistent block-device %s (%Lu)\n",
                        bdevname(bio->bi_bdev, b),
                        (long long) bio->bi_sector);
end_io:
                  bio_endio(bio, err);
                  break;
            }

            if (unlikely(nr_sectors > q->max_hw_sectors)) {
                  printk("bio too big device %s (%u > %u)\n", 
                        bdevname(bio->bi_bdev, b),
                        bio_sectors(bio),
                        q->max_hw_sectors);
                  goto end_io;
            }

            if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
                  goto end_io;

            if (should_fail_request(bio))
                  goto end_io;

            /*
             * If this device has partitions, remap block n
             * of partition p to block n+start(p) of the disk.
             */
            blk_partition_remap(bio);

            if (old_sector != -1)
                  blk_add_trace_remap(q, bio, old_dev, bio->bi_sector,
                                  old_sector);

            blk_add_trace_bio(q, bio, BLK_TA_QUEUE);

            old_sector = bio->bi_sector;
            old_dev = bio->bi_bdev->bd_dev;

            if (bio_check_eod(bio, nr_sectors))
                  goto end_io;
            if (bio_empty_barrier(bio) && !q->prepare_flush_fn) {
                  err = -EOPNOTSUPP;
                  goto end_io;
            }

            ret = q->make_request_fn(q, bio);
      } while (ret);
}

/*
 * We only want one ->make_request_fn to be active at a time,
 * else stack usage with stacked devices could be a problem.
 * So use current->bio_{list,tail} to keep a list of requests
 * submited by a make_request_fn function.
 * current->bio_tail is also used as a flag to say if
 * generic_make_request is currently active in this task or not.
 * If it is NULL, then no make_request is active.  If it is non-NULL,
 * then a make_request is active, and new requests should be added
 * at the tail
 */
void generic_make_request(struct bio *bio)
{
      if (current->bio_tail) {
            /* make_request is active */
            *(current->bio_tail) = bio;
            bio->bi_next = NULL;
            current->bio_tail = &bio->bi_next;
            return;
      }
      /* following loop may be a bit non-obvious, and so deserves some
       * explanation.
       * Before entering the loop, bio->bi_next is NULL (as all callers
       * ensure that) so we have a list with a single bio.
       * We pretend that we have just taken it off a longer list, so
       * we assign bio_list to the next (which is NULL) and bio_tail
       * to &bio_list, thus initialising the bio_list of new bios to be
       * added.  __generic_make_request may indeed add some more bios
       * through a recursive call to generic_make_request.  If it
       * did, we find a non-NULL value in bio_list and re-enter the loop
       * from the top.  In this case we really did just take the bio
       * of the top of the list (no pretending) and so fixup bio_list and
       * bio_tail or bi_next, and call into __generic_make_request again.
       *
       * The loop was structured like this to make only one call to
       * __generic_make_request (which is important as it is large and
       * inlined) and to keep the structure simple.
       */
      BUG_ON(bio->bi_next);
      do {
            current->bio_list = bio->bi_next;
            if (bio->bi_next == NULL)
                  current->bio_tail = &current->bio_list;
            else
                  bio->bi_next = NULL;
            __generic_make_request(bio);
            bio = current->bio_list;
      } while (bio);
      current->bio_tail = NULL; /* deactivate */
}

EXPORT_SYMBOL(generic_make_request);

/**
 * submit_bio: submit a bio to the block device layer for I/O
 * @rw: whether to %READ or %WRITE, or maybe to %READA (read ahead)
 * @bio: The &struct bio which describes the I/O
 *
 * submit_bio() is very similar in purpose to generic_make_request(), and
 * uses that function to do most of the work. Both are fairly rough
 * interfaces, @bio must be presetup and ready for I/O.
 *
 */
void submit_bio(int rw, struct bio *bio)
{
      int count = bio_sectors(bio);

      bio->bi_rw |= rw;

      /*
       * If it's a regular read/write or a barrier with data attached,
       * go through the normal accounting stuff before submission.
       */
      if (!bio_empty_barrier(bio)) {

            BIO_BUG_ON(!bio->bi_size);
            BIO_BUG_ON(!bio->bi_io_vec);

            if (rw & WRITE) {
                  count_vm_events(PGPGOUT, count);
            } else {
                  task_io_account_read(bio->bi_size);
                  count_vm_events(PGPGIN, count);
            }

            if (unlikely(block_dump)) {
                  char b[BDEVNAME_SIZE];
                  printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
                  current->comm, task_pid_nr(current),
                        (rw & WRITE) ? "WRITE" : "READ",
                        (unsigned long long)bio->bi_sector,
                        bdevname(bio->bi_bdev,b));
            }
      }

      generic_make_request(bio);
}

EXPORT_SYMBOL(submit_bio);

static void blk_recalc_rq_sectors(struct request *rq, int nsect)
{
      if (blk_fs_request(rq)) {
            rq->hard_sector += nsect;
            rq->hard_nr_sectors -= nsect;

            /*
             * Move the I/O submission pointers ahead if required.
             */
            if ((rq->nr_sectors >= rq->hard_nr_sectors) &&
                (rq->sector <= rq->hard_sector)) {
                  rq->sector = rq->hard_sector;
                  rq->nr_sectors = rq->hard_nr_sectors;
                  rq->hard_cur_sectors = bio_cur_sectors(rq->bio);
                  rq->current_nr_sectors = rq->hard_cur_sectors;
                  rq->buffer = bio_data(rq->bio);
            }

            /*
             * if total number of sectors is less than the first segment
             * size, something has gone terribly wrong
             */
            if (rq->nr_sectors < rq->current_nr_sectors) {
                  printk("blk: request botched\n");
                  rq->nr_sectors = rq->current_nr_sectors;
            }
      }
}

static int __end_that_request_first(struct request *req, int uptodate,
                            int nr_bytes)
{
      int total_bytes, bio_nbytes, error, next_idx = 0;
      struct bio *bio;

      blk_add_trace_rq(req->q, req, BLK_TA_COMPLETE);

      /*
       * extend uptodate bool to allow < 0 value to be direct io error
       */
      error = 0;
      if (end_io_error(uptodate))
            error = !uptodate ? -EIO : uptodate;

      /*
       * for a REQ_BLOCK_PC request, we want to carry any eventual
       * sense key with us all the way through
       */
      if (!blk_pc_request(req))
            req->errors = 0;

      if (!uptodate) {
            if (blk_fs_request(req) && !(req->cmd_flags & REQ_QUIET))
                  printk("end_request: I/O error, dev %s, sector %llu\n",
                        req->rq_disk ? req->rq_disk->disk_name : "?",
                        (unsigned long long)req->sector);
      }

      if (blk_fs_request(req) && req->rq_disk) {
            const int rw = rq_data_dir(req);

            disk_stat_add(req->rq_disk, sectors[rw], nr_bytes >> 9);
      }

      total_bytes = bio_nbytes = 0;
      while ((bio = req->bio) != NULL) {
            int nbytes;

            /*
             * For an empty barrier request, the low level driver must
             * store a potential error location in ->sector. We pass
             * that back up in ->bi_sector.
             */
            if (blk_empty_barrier(req))
                  bio->bi_sector = req->sector;

            if (nr_bytes >= bio->bi_size) {
                  req->bio = bio->bi_next;
                  nbytes = bio->bi_size;
                  req_bio_endio(req, bio, nbytes, error);
                  next_idx = 0;
                  bio_nbytes = 0;
            } else {
                  int idx = bio->bi_idx + next_idx;

                  if (unlikely(bio->bi_idx >= bio->bi_vcnt)) {
                        blk_dump_rq_flags(req, "__end_that");
                        printk("%s: bio idx %d >= vcnt %d\n",
                                    __FUNCTION__,
                                    bio->bi_idx, bio->bi_vcnt);
                        break;
                  }

                  nbytes = bio_iovec_idx(bio, idx)->bv_len;
                  BIO_BUG_ON(nbytes > bio->bi_size);

                  /*
                   * not a complete bvec done
                   */
                  if (unlikely(nbytes > nr_bytes)) {
                        bio_nbytes += nr_bytes;
                        total_bytes += nr_bytes;
                        break;
                  }

                  /*
                   * advance to the next vector
                   */
                  next_idx++;
                  bio_nbytes += nbytes;
            }

            total_bytes += nbytes;
            nr_bytes -= nbytes;

            if ((bio = req->bio)) {
                  /*
                   * end more in this run, or just return 'not-done'
                   */
                  if (unlikely(nr_bytes <= 0))
                        break;
            }
      }

      /*
       * completely done
       */
      if (!req->bio)
            return 0;

      /*
       * if the request wasn't completed, update state
       */
      if (bio_nbytes) {
            req_bio_endio(req, bio, bio_nbytes, error);
            bio->bi_idx += next_idx;
            bio_iovec(bio)->bv_offset += nr_bytes;
            bio_iovec(bio)->bv_len -= nr_bytes;
      }

      blk_recalc_rq_sectors(req, total_bytes >> 9);
      blk_recalc_rq_segments(req);
      return 1;
}

/**
 * end_that_request_first - end I/O on a request
 * @req:      the request being processed
 * @uptodate: 1 for success, 0 for I/O error, < 0 for specific error
 * @nr_sectors: number of sectors to end I/O on
 *
 * Description:
 *     Ends I/O on a number of sectors attached to @req, and sets it up
 *     for the next range of segments (if any) in the cluster.
 *
 * Return:
 *     0 - we are done with this request, call end_that_request_last()
 *     1 - still buffers pending for this request
 **/
int end_that_request_first(struct request *req, int uptodate, int nr_sectors)
{
      return __end_that_request_first(req, uptodate, nr_sectors << 9);
}

EXPORT_SYMBOL(end_that_request_first);

/**
 * end_that_request_chunk - end I/O on a request
 * @req:      the request being processed
 * @uptodate: 1 for success, 0 for I/O error, < 0 for specific error
 * @nr_bytes: number of bytes to complete
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @req, and sets it up
 *     for the next range of segments (if any). Like end_that_request_first(),
 *     but deals with bytes instead of sectors.
 *
 * Return:
 *     0 - we are done with this request, call end_that_request_last()
 *     1 - still buffers pending for this request
 **/
int end_that_request_chunk(struct request *req, int uptodate, int nr_bytes)
{
      return __end_that_request_first(req, uptodate, nr_bytes);
}

EXPORT_SYMBOL(end_that_request_chunk);

/*
 * splice the completion data to a local structure and hand off to
 * process_completion_queue() to complete the requests
 */
static void blk_done_softirq(struct softirq_action *h)
{
      struct list_head *cpu_list, local_list;

      local_irq_disable();
      cpu_list = &__get_cpu_var(blk_cpu_done);
      list_replace_init(cpu_list, &local_list);
      local_irq_enable();

      while (!list_empty(&local_list)) {
            struct request *rq = list_entry(local_list.next, struct request, donelist);

            list_del_init(&rq->donelist);
            rq->q->softirq_done_fn(rq);
      }
}

static int __cpuinit blk_cpu_notify(struct notifier_block *self, unsigned long action,
                    void *hcpu)
{
      /*
       * If a CPU goes away, splice its entries to the current CPU
       * and trigger a run of the softirq
       */
      if (action == CPU_DEAD || action == CPU_DEAD_FROZEN) {
            int cpu = (unsigned long) hcpu;

            local_irq_disable();
            list_splice_init(&per_cpu(blk_cpu_done, cpu),
                         &__get_cpu_var(blk_cpu_done));
            raise_softirq_irqoff(BLOCK_SOFTIRQ);
            local_irq_enable();
      }

      return NOTIFY_OK;
}


static struct notifier_block blk_cpu_notifier __cpuinitdata = {
      .notifier_call    = blk_cpu_notify,
};

/**
 * blk_complete_request - end I/O on a request
 * @req:      the request being processed
 *
 * Description:
 *     Ends all I/O on a request. It does not handle partial completions,
 *     unless the driver actually implements this in its completion callback
 *     through requeueing. The actual completion happens out-of-order,
 *     through a softirq handler. The user must have registered a completion
 *     callback through blk_queue_softirq_done().
 **/

void blk_complete_request(struct request *req)
{
      struct list_head *cpu_list;
      unsigned long flags;

      BUG_ON(!req->q->softirq_done_fn);
            
      local_irq_save(flags);

      cpu_list = &__get_cpu_var(blk_cpu_done);
      list_add_tail(&req->donelist, cpu_list);
      raise_softirq_irqoff(BLOCK_SOFTIRQ);

      local_irq_restore(flags);
}

EXPORT_SYMBOL(blk_complete_request);
      
/*
 * queue lock must be held
 */
void end_that_request_last(struct request *req, int uptodate)
{
      struct gendisk *disk = req->rq_disk;
      int error;

      /*
       * extend uptodate bool to allow < 0 value to be direct io error
       */
      error = 0;
      if (end_io_error(uptodate))
            error = !uptodate ? -EIO : uptodate;

      if (unlikely(laptop_mode) && blk_fs_request(req))
            laptop_io_completion();

      /*
       * Account IO completion.  bar_rq isn't accounted as a normal
       * IO on queueing nor completion.  Accounting the containing
       * request is enough.
       */
      if (disk && blk_fs_request(req) && req != &req->q->bar_rq) {
            unsigned long duration = jiffies - req->start_time;
            const int rw = rq_data_dir(req);

            __disk_stat_inc(disk, ios[rw]);
            __disk_stat_add(disk, ticks[rw], duration);
            disk_round_stats(disk);
            disk->in_flight--;
      }
      if (req->end_io)
            req->end_io(req, error);
      else
            __blk_put_request(req->q, req);
}

EXPORT_SYMBOL(end_that_request_last);

static inline void __end_request(struct request *rq, int uptodate,
                         unsigned int nr_bytes, int dequeue)
{
      if (!end_that_request_chunk(rq, uptodate, nr_bytes)) {
            if (dequeue)
                  blkdev_dequeue_request(rq);
            add_disk_randomness(rq->rq_disk);
            end_that_request_last(rq, uptodate);
      }
}

static unsigned int rq_byte_size(struct request *rq)
{
      if (blk_fs_request(rq))
            return rq->hard_nr_sectors << 9;

      return rq->data_len;
}

/**
 * end_queued_request - end all I/O on a queued request
 * @rq:           the request being processed
 * @uptodate:     error value or 0/1 uptodate flag
 *
 * Description:
 *     Ends all I/O on a request, and removes it from the block layer queues.
 *     Not suitable for normal IO completion, unless the driver still has
 *     the request attached to the block layer.
 *
 **/
void end_queued_request(struct request *rq, int uptodate)
{
      __end_request(rq, uptodate, rq_byte_size(rq), 1);
}
EXPORT_SYMBOL(end_queued_request);

/**
 * end_dequeued_request - end all I/O on a dequeued request
 * @rq:           the request being processed
 * @uptodate:     error value or 0/1 uptodate flag
 *
 * Description:
 *     Ends all I/O on a request. The request must already have been
 *     dequeued using blkdev_dequeue_request(), as is normally the case
 *     for most drivers.
 *
 **/
void end_dequeued_request(struct request *rq, int uptodate)
{
      __end_request(rq, uptodate, rq_byte_size(rq), 0);
}
EXPORT_SYMBOL(end_dequeued_request);


/**
 * end_request - end I/O on the current segment of the request
 * @req:    the request being processed
 * @uptodate:     error value or 0/1 uptodate flag
 *
 * Description:
 *     Ends I/O on the current segment of a request. If that is the only
 *     remaining segment, the request is also completed and freed.
 *
 *     This is a remnant of how older block drivers handled IO completions.
 *     Modern drivers typically end IO on the full request in one go, unless
 *     they have a residual value to account for. For that case this function
 *     isn't really useful, unless the residual just happens to be the
 *     full current segment. In other words, don't use this function in new
 *     code. Either use end_request_completely(), or the
 *     end_that_request_chunk() (along with end_that_request_last()) for
 *     partial completions.
 *
 **/
void end_request(struct request *req, int uptodate)
{
      __end_request(req, uptodate, req->hard_cur_sectors << 9, 1);
}
EXPORT_SYMBOL(end_request);

static void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
                      struct bio *bio)
{
      /* first two bits are identical in rq->cmd_flags and bio->bi_rw */
      rq->cmd_flags |= (bio->bi_rw & 3);

      rq->nr_phys_segments = bio_phys_segments(q, bio);
      rq->nr_hw_segments = bio_hw_segments(q, bio);
      rq->current_nr_sectors = bio_cur_sectors(bio);
      rq->hard_cur_sectors = rq->current_nr_sectors;
      rq->hard_nr_sectors = rq->nr_sectors = bio_sectors(bio);
      rq->buffer = bio_data(bio);
      rq->data_len = bio->bi_size;

      rq->bio = rq->biotail = bio;

      if (bio->bi_bdev)
            rq->rq_disk = bio->bi_bdev->bd_disk;
}

int kblockd_schedule_work(struct work_struct *work)
{
      return queue_work(kblockd_workqueue, work);
}

EXPORT_SYMBOL(kblockd_schedule_work);

void kblockd_flush_work(struct work_struct *work)
{
      cancel_work_sync(work);
}
EXPORT_SYMBOL(kblockd_flush_work);

int __init blk_dev_init(void)
{
      int i;

      kblockd_workqueue = create_workqueue("kblockd");
      if (!kblockd_workqueue)
            panic("Failed to create kblockd\n");

      request_cachep = kmem_cache_create("blkdev_requests",
                  sizeof(struct request), 0, SLAB_PANIC, NULL);

      requestq_cachep = kmem_cache_create("blkdev_queue",
                  sizeof(struct request_queue), 0, SLAB_PANIC, NULL);

      iocontext_cachep = kmem_cache_create("blkdev_ioc",
                  sizeof(struct io_context), 0, SLAB_PANIC, NULL);

      for_each_possible_cpu(i)
            INIT_LIST_HEAD(&per_cpu(blk_cpu_done, i));

      open_softirq(BLOCK_SOFTIRQ, blk_done_softirq, NULL);
      register_hotcpu_notifier(&blk_cpu_notifier);

      blk_max_low_pfn = max_low_pfn - 1;
      blk_max_pfn = max_pfn - 1;

      return 0;
}

/*
 * IO Context helper functions
 */
void put_io_context(struct io_context *ioc)
{
      if (ioc == NULL)
            return;

      BUG_ON(atomic_read(&ioc->refcount) == 0);

      if (atomic_dec_and_test(&ioc->refcount)) {
            struct cfq_io_context *cic;

            rcu_read_lock();
            if (ioc->aic && ioc->aic->dtor)
                  ioc->aic->dtor(ioc->aic);
            if (ioc->cic_root.rb_node != NULL) {
                  struct rb_node *n = rb_first(&ioc->cic_root);

                  cic = rb_entry(n, struct cfq_io_context, rb_node);
                  cic->dtor(ioc);
            }
            rcu_read_unlock();

            kmem_cache_free(iocontext_cachep, ioc);
      }
}
EXPORT_SYMBOL(put_io_context);

/* Called by the exitting task */
void exit_io_context(void)
{
      struct io_context *ioc;
      struct cfq_io_context *cic;

      task_lock(current);
      ioc = current->io_context;
      current->io_context = NULL;
      task_unlock(current);

      ioc->task = NULL;
      if (ioc->aic && ioc->aic->exit)
            ioc->aic->exit(ioc->aic);
      if (ioc->cic_root.rb_node != NULL) {
            cic = rb_entry(rb_first(&ioc->cic_root), struct cfq_io_context, rb_node);
            cic->exit(ioc);
      }

      put_io_context(ioc);
}

/*
 * If the current task has no IO context then create one and initialise it.
 * Otherwise, return its existing IO context.
 *
 * This returned IO context doesn't have a specifically elevated refcount,
 * but since the current task itself holds a reference, the context can be
 * used in general code, so long as it stays within `current` context.
 */
static struct io_context *current_io_context(gfp_t gfp_flags, int node)
{
      struct task_struct *tsk = current;
      struct io_context *ret;

      ret = tsk->io_context;
      if (likely(ret))
            return ret;

      ret = kmem_cache_alloc_node(iocontext_cachep, gfp_flags, node);
      if (ret) {
            atomic_set(&ret->refcount, 1);
            ret->task = current;
            ret->ioprio_changed = 0;
            ret->last_waited = jiffies; /* doesn't matter... */
            ret->nr_batch_requests = 0; /* because this is 0 */
            ret->aic = NULL;
            ret->cic_root.rb_node = NULL;
            ret->ioc_data = NULL;
            /* make sure set_task_ioprio() sees the settings above */
            smp_wmb();
            tsk->io_context = ret;
      }

      return ret;
}

/*
 * If the current task has no IO context then create one and initialise it.
 * If it does have a context, take a ref on it.
 *
 * This is always called in the context of the task which submitted the I/O.
 */
struct io_context *get_io_context(gfp_t gfp_flags, int node)
{
      struct io_context *ret;
      ret = current_io_context(gfp_flags, node);
      if (likely(ret))
            atomic_inc(&ret->refcount);
      return ret;
}
EXPORT_SYMBOL(get_io_context);

void copy_io_context(struct io_context **pdst, struct io_context **psrc)
{
      struct io_context *src = *psrc;
      struct io_context *dst = *pdst;

      if (src) {
            BUG_ON(atomic_read(&src->refcount) == 0);
            atomic_inc(&src->refcount);
            put_io_context(dst);
            *pdst = src;
      }
}
EXPORT_SYMBOL(copy_io_context);

void swap_io_context(struct io_context **ioc1, struct io_context **ioc2)
{
      struct io_context *temp;
      temp = *ioc1;
      *ioc1 = *ioc2;
      *ioc2 = temp;
}
EXPORT_SYMBOL(swap_io_context);

/*
 * sysfs parts below
 */
struct queue_sysfs_entry {
      struct attribute attr;
      ssize_t (*show)(struct request_queue *, char *);
      ssize_t (*store)(struct request_queue *, const char *, size_t);
};

static ssize_t
queue_var_show(unsigned int var, char *page)
{
      return sprintf(page, "%d\n", var);
}

static ssize_t
queue_var_store(unsigned long *var, const char *page, size_t count)
{
      char *p = (char *) page;

      *var = simple_strtoul(p, &p, 10);
      return count;
}

static ssize_t queue_requests_show(struct request_queue *q, char *page)
{
      return queue_var_show(q->nr_requests, (page));
}

static ssize_t
queue_requests_store(struct request_queue *q, const char *page, size_t count)
{
      struct request_list *rl = &q->rq;
      unsigned long nr;
      int ret = queue_var_store(&nr, page, count);
      if (nr < BLKDEV_MIN_RQ)
            nr = BLKDEV_MIN_RQ;

      spin_lock_irq(q->queue_lock);
      q->nr_requests = nr;
      blk_queue_congestion_threshold(q);

      if (rl->count[READ] >= queue_congestion_on_threshold(q))
            blk_set_queue_congested(q, READ);
      else if (rl->count[READ] < queue_congestion_off_threshold(q))
            blk_clear_queue_congested(q, READ);

      if (rl->count[WRITE] >= queue_congestion_on_threshold(q))
            blk_set_queue_congested(q, WRITE);
      else if (rl->count[WRITE] < queue_congestion_off_threshold(q))
            blk_clear_queue_congested(q, WRITE);

      if (rl->count[READ] >= q->nr_requests) {
            blk_set_queue_full(q, READ);
      } else if (rl->count[READ]+1 <= q->nr_requests) {
            blk_clear_queue_full(q, READ);
            wake_up(&rl->wait[READ]);
      }

      if (rl->count[WRITE] >= q->nr_requests) {
            blk_set_queue_full(q, WRITE);
      } else if (rl->count[WRITE]+1 <= q->nr_requests) {
            blk_clear_queue_full(q, WRITE);
            wake_up(&rl->wait[WRITE]);
      }
      spin_unlock_irq(q->queue_lock);
      return ret;
}

static ssize_t queue_ra_show(struct request_queue *q, char *page)
{
      int ra_kb = q->backing_dev_info.ra_pages << (PAGE_CACHE_SHIFT - 10);

      return queue_var_show(ra_kb, (page));
}

static ssize_t
queue_ra_store(struct request_queue *q, const char *page, size_t count)
{
      unsigned long ra_kb;
      ssize_t ret = queue_var_store(&ra_kb, page, count);

      spin_lock_irq(q->queue_lock);
      q->backing_dev_info.ra_pages = ra_kb >> (PAGE_CACHE_SHIFT - 10);
      spin_unlock_irq(q->queue_lock);

      return ret;
}

static ssize_t queue_max_sectors_show(struct request_queue *q, char *page)
{
      int max_sectors_kb = q->max_sectors >> 1;

      return queue_var_show(max_sectors_kb, (page));
}

static ssize_t
queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
{
      unsigned long max_sectors_kb,
                  max_hw_sectors_kb = q->max_hw_sectors >> 1,
                  page_kb = 1 << (PAGE_CACHE_SHIFT - 10);
      ssize_t ret = queue_var_store(&max_sectors_kb, page, count);

      if (max_sectors_kb > max_hw_sectors_kb || max_sectors_kb < page_kb)
            return -EINVAL;
      /*
       * Take the queue lock to update the readahead and max_sectors
       * values synchronously:
       */
      spin_lock_irq(q->queue_lock);
      q->max_sectors = max_sectors_kb << 1;
      spin_unlock_irq(q->queue_lock);

      return ret;
}

static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)
{
      int max_hw_sectors_kb = q->max_hw_sectors >> 1;

      return queue_var_show(max_hw_sectors_kb, (page));
}


static struct queue_sysfs_entry queue_requests_entry = {
      .attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
      .show = queue_requests_show,
      .store = queue_requests_store,
};

static struct queue_sysfs_entry queue_ra_entry = {
      .attr = {.name = "read_ahead_kb", .mode = S_IRUGO | S_IWUSR },
      .show = queue_ra_show,
      .store = queue_ra_store,
};

static struct queue_sysfs_entry queue_max_sectors_entry = {
      .attr = {.name = "max_sectors_kb", .mode = S_IRUGO | S_IWUSR },
      .show = queue_max_sectors_show,
      .store = queue_max_sectors_store,
};

static struct queue_sysfs_entry queue_max_hw_sectors_entry = {
      .attr = {.name = "max_hw_sectors_kb", .mode = S_IRUGO },
      .show = queue_max_hw_sectors_show,
};

static struct queue_sysfs_entry queue_iosched_entry = {
      .attr = {.name = "scheduler", .mode = S_IRUGO | S_IWUSR },
      .show = elv_iosched_show,
      .store = elv_iosched_store,
};

static struct attribute *default_attrs[] = {
      &queue_requests_entry.attr,
      &queue_ra_entry.attr,
      &queue_max_hw_sectors_entry.attr,
      &queue_max_sectors_entry.attr,
      &queue_iosched_entry.attr,
      NULL,
};

#define to_queue(atr) container_of((atr), struct queue_sysfs_entry, attr)

static ssize_t
queue_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
{
      struct queue_sysfs_entry *entry = to_queue(attr);
      struct request_queue *q =
            container_of(kobj, struct request_queue, kobj);
      ssize_t res;

      if (!entry->show)
            return -EIO;
      mutex_lock(&q->sysfs_lock);
      if (test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)) {
            mutex_unlock(&q->sysfs_lock);
            return -ENOENT;
      }
      res = entry->show(q, page);
      mutex_unlock(&q->sysfs_lock);
      return res;
}

static ssize_t
queue_attr_store(struct kobject *kobj, struct attribute *attr,
                const char *page, size_t length)
{
      struct queue_sysfs_entry *entry = to_queue(attr);
      struct request_queue *q = container_of(kobj, struct request_queue, kobj);

      ssize_t res;

      if (!entry->store)
            return -EIO;
      mutex_lock(&q->sysfs_lock);
      if (test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)) {
            mutex_unlock(&q->sysfs_lock);
            return -ENOENT;
      }
      res = entry->store(q, page, length);
      mutex_unlock(&q->sysfs_lock);
      return res;
}

static struct sysfs_ops queue_sysfs_ops = {
      .show = queue_attr_show,
      .store      = queue_attr_store,
};

static struct kobj_type queue_ktype = {
      .sysfs_ops  = &queue_sysfs_ops,
      .default_attrs    = default_attrs,
      .release    = blk_release_queue,
};

int blk_register_queue(struct gendisk *disk)
{
      int ret;

      struct request_queue *q = disk->queue;

      if (!q || !q->request_fn)
            return -ENXIO;

      q->kobj.parent = kobject_get(&disk->kobj);

      ret = kobject_add(&q->kobj);
      if (ret < 0)
            return ret;

      kobject_uevent(&q->kobj, KOBJ_ADD);

      ret = elv_register_queue(q);
      if (ret) {
            kobject_uevent(&q->kobj, KOBJ_REMOVE);
            kobject_del(&q->kobj);
            return ret;
      }

      return 0;
}

void blk_unregister_queue(struct gendisk *disk)
{
      struct request_queue *q = disk->queue;

      if (q && q->request_fn) {
            elv_unregister_queue(q);

            kobject_uevent(&q->kobj, KOBJ_REMOVE);
            kobject_del(&q->kobj);
            kobject_put(&disk->kobj);
      }
}

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