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dm-snap.c

/*
 * dm-snapshot.c
 *
 * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
 *
 * This file is released under the GPL.
 */

#include <linux/blkdev.h>
#include <linux/ctype.h>
#include <linux/device-mapper.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kdev_t.h>
#include <linux/list.h>
#include <linux/mempool.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/log2.h>

#include "dm-snap.h"
#include "dm-bio-list.h"
#include "kcopyd.h"

#define DM_MSG_PREFIX "snapshots"

/*
 * The percentage increment we will wake up users at
 */
#define WAKE_UP_PERCENT 5

/*
 * kcopyd priority of snapshot operations
 */
#define SNAPSHOT_COPY_PRIORITY 2

/*
 * Each snapshot reserves this many pages for io
 */
#define SNAPSHOT_PAGES 256

static struct workqueue_struct *ksnapd;
static void flush_queued_bios(struct work_struct *work);

struct dm_snap_pending_exception {
      struct dm_snap_exception e;

      /*
       * Origin buffers waiting for this to complete are held
       * in a bio list
       */
      struct bio_list origin_bios;
      struct bio_list snapshot_bios;

      /*
       * Short-term queue of pending exceptions prior to submission.
       */
      struct list_head list;

      /*
       * The primary pending_exception is the one that holds
       * the ref_count and the list of origin_bios for a
       * group of pending_exceptions.  It is always last to get freed.
       * These fields get set up when writing to the origin.
       */
      struct dm_snap_pending_exception *primary_pe;

      /*
       * Number of pending_exceptions processing this chunk.
       * When this drops to zero we must complete the origin bios.
       * If incrementing or decrementing this, hold pe->snap->lock for
       * the sibling concerned and not pe->primary_pe->snap->lock unless
       * they are the same.
       */
      atomic_t ref_count;

      /* Pointer back to snapshot context */
      struct dm_snapshot *snap;

      /*
       * 1 indicates the exception has already been sent to
       * kcopyd.
       */
      int started;
};

/*
 * Hash table mapping origin volumes to lists of snapshots and
 * a lock to protect it
 */
static struct kmem_cache *exception_cache;
static struct kmem_cache *pending_cache;
static mempool_t *pending_pool;

/*
 * One of these per registered origin, held in the snapshot_origins hash
 */
struct origin {
      /* The origin device */
      struct block_device *bdev;

      struct list_head hash_list;

      /* List of snapshots for this origin */
      struct list_head snapshots;
};

/*
 * Size of the hash table for origin volumes. If we make this
 * the size of the minors list then it should be nearly perfect
 */
#define ORIGIN_HASH_SIZE 256
#define ORIGIN_MASK      0xFF
static struct list_head *_origins;
static struct rw_semaphore _origins_lock;

static int init_origin_hash(void)
{
      int i;

      _origins = kmalloc(ORIGIN_HASH_SIZE * sizeof(struct list_head),
                     GFP_KERNEL);
      if (!_origins) {
            DMERR("unable to allocate memory");
            return -ENOMEM;
      }

      for (i = 0; i < ORIGIN_HASH_SIZE; i++)
            INIT_LIST_HEAD(_origins + i);
      init_rwsem(&_origins_lock);

      return 0;
}

static void exit_origin_hash(void)
{
      kfree(_origins);
}

static unsigned origin_hash(struct block_device *bdev)
{
      return bdev->bd_dev & ORIGIN_MASK;
}

static struct origin *__lookup_origin(struct block_device *origin)
{
      struct list_head *ol;
      struct origin *o;

      ol = &_origins[origin_hash(origin)];
      list_for_each_entry (o, ol, hash_list)
            if (bdev_equal(o->bdev, origin))
                  return o;

      return NULL;
}

static void __insert_origin(struct origin *o)
{
      struct list_head *sl = &_origins[origin_hash(o->bdev)];
      list_add_tail(&o->hash_list, sl);
}

/*
 * Make a note of the snapshot and its origin so we can look it
 * up when the origin has a write on it.
 */
static int register_snapshot(struct dm_snapshot *snap)
{
      struct origin *o;
      struct block_device *bdev = snap->origin->bdev;

      down_write(&_origins_lock);
      o = __lookup_origin(bdev);

      if (!o) {
            /* New origin */
            o = kmalloc(sizeof(*o), GFP_KERNEL);
            if (!o) {
                  up_write(&_origins_lock);
                  return -ENOMEM;
            }

            /* Initialise the struct */
            INIT_LIST_HEAD(&o->snapshots);
            o->bdev = bdev;

            __insert_origin(o);
      }

      list_add_tail(&snap->list, &o->snapshots);

      up_write(&_origins_lock);
      return 0;
}

static void unregister_snapshot(struct dm_snapshot *s)
{
      struct origin *o;

      down_write(&_origins_lock);
      o = __lookup_origin(s->origin->bdev);

      list_del(&s->list);
      if (list_empty(&o->snapshots)) {
            list_del(&o->hash_list);
            kfree(o);
      }

      up_write(&_origins_lock);
}

/*
 * Implementation of the exception hash tables.
 */
static int init_exception_table(struct exception_table *et, uint32_t size)
{
      unsigned int i;

      et->hash_mask = size - 1;
      et->table = dm_vcalloc(size, sizeof(struct list_head));
      if (!et->table)
            return -ENOMEM;

      for (i = 0; i < size; i++)
            INIT_LIST_HEAD(et->table + i);

      return 0;
}

static void exit_exception_table(struct exception_table *et, struct kmem_cache *mem)
{
      struct list_head *slot;
      struct dm_snap_exception *ex, *next;
      int i, size;

      size = et->hash_mask + 1;
      for (i = 0; i < size; i++) {
            slot = et->table + i;

            list_for_each_entry_safe (ex, next, slot, hash_list)
                  kmem_cache_free(mem, ex);
      }

      vfree(et->table);
}

static uint32_t exception_hash(struct exception_table *et, chunk_t chunk)
{
      return chunk & et->hash_mask;
}

static void insert_exception(struct exception_table *eh,
                       struct dm_snap_exception *e)
{
      struct list_head *l = &eh->table[exception_hash(eh, e->old_chunk)];
      list_add(&e->hash_list, l);
}

static void remove_exception(struct dm_snap_exception *e)
{
      list_del(&e->hash_list);
}

/*
 * Return the exception data for a sector, or NULL if not
 * remapped.
 */
static struct dm_snap_exception *lookup_exception(struct exception_table *et,
                                      chunk_t chunk)
{
      struct list_head *slot;
      struct dm_snap_exception *e;

      slot = &et->table[exception_hash(et, chunk)];
      list_for_each_entry (e, slot, hash_list)
            if (e->old_chunk == chunk)
                  return e;

      return NULL;
}

static struct dm_snap_exception *alloc_exception(void)
{
      struct dm_snap_exception *e;

      e = kmem_cache_alloc(exception_cache, GFP_NOIO);
      if (!e)
            e = kmem_cache_alloc(exception_cache, GFP_ATOMIC);

      return e;
}

static void free_exception(struct dm_snap_exception *e)
{
      kmem_cache_free(exception_cache, e);
}

static struct dm_snap_pending_exception *alloc_pending_exception(void)
{
      return mempool_alloc(pending_pool, GFP_NOIO);
}

static void free_pending_exception(struct dm_snap_pending_exception *pe)
{
      mempool_free(pe, pending_pool);
}

int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new)
{
      struct dm_snap_exception *e;

      e = alloc_exception();
      if (!e)
            return -ENOMEM;

      e->old_chunk = old;
      e->new_chunk = new;
      insert_exception(&s->complete, e);
      return 0;
}

/*
 * Hard coded magic.
 */
static int calc_max_buckets(void)
{
      /* use a fixed size of 2MB */
      unsigned long mem = 2 * 1024 * 1024;
      mem /= sizeof(struct list_head);

      return mem;
}

/*
 * Rounds a number down to a power of 2.
 */
static uint32_t round_down(uint32_t n)
{
      while (n & (n - 1))
            n &= (n - 1);
      return n;
}

/*
 * Allocate room for a suitable hash table.
 */
static int init_hash_tables(struct dm_snapshot *s)
{
      sector_t hash_size, cow_dev_size, origin_dev_size, max_buckets;

      /*
       * Calculate based on the size of the original volume or
       * the COW volume...
       */
      cow_dev_size = get_dev_size(s->cow->bdev);
      origin_dev_size = get_dev_size(s->origin->bdev);
      max_buckets = calc_max_buckets();

      hash_size = min(origin_dev_size, cow_dev_size) >> s->chunk_shift;
      hash_size = min(hash_size, max_buckets);

      /* Round it down to a power of 2 */
      hash_size = round_down(hash_size);
      if (init_exception_table(&s->complete, hash_size))
            return -ENOMEM;

      /*
       * Allocate hash table for in-flight exceptions
       * Make this smaller than the real hash table
       */
      hash_size >>= 3;
      if (hash_size < 64)
            hash_size = 64;

      if (init_exception_table(&s->pending, hash_size)) {
            exit_exception_table(&s->complete, exception_cache);
            return -ENOMEM;
      }

      return 0;
}

/*
 * Round a number up to the nearest 'size' boundary.  size must
 * be a power of 2.
 */
static ulong round_up(ulong n, ulong size)
{
      size--;
      return (n + size) & ~size;
}

static int set_chunk_size(struct dm_snapshot *s, const char *chunk_size_arg,
                    char **error)
{
      unsigned long chunk_size;
      char *value;

      chunk_size = simple_strtoul(chunk_size_arg, &value, 10);
      if (*chunk_size_arg == '\0' || *value != '\0') {
            *error = "Invalid chunk size";
            return -EINVAL;
      }

      if (!chunk_size) {
            s->chunk_size = s->chunk_mask = s->chunk_shift = 0;
            return 0;
      }

      /*
       * Chunk size must be multiple of page size.  Silently
       * round up if it's not.
       */
      chunk_size = round_up(chunk_size, PAGE_SIZE >> 9);

      /* Check chunk_size is a power of 2 */
      if (!is_power_of_2(chunk_size)) {
            *error = "Chunk size is not a power of 2";
            return -EINVAL;
      }

      /* Validate the chunk size against the device block size */
      if (chunk_size % (bdev_hardsect_size(s->cow->bdev) >> 9)) {
            *error = "Chunk size is not a multiple of device blocksize";
            return -EINVAL;
      }

      s->chunk_size = chunk_size;
      s->chunk_mask = chunk_size - 1;
      s->chunk_shift = ffs(chunk_size) - 1;

      return 0;
}

/*
 * Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
 */
static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
      struct dm_snapshot *s;
      int r = -EINVAL;
      char persistent;
      char *origin_path;
      char *cow_path;

      if (argc != 4) {
            ti->error = "requires exactly 4 arguments";
            r = -EINVAL;
            goto bad1;
      }

      origin_path = argv[0];
      cow_path = argv[1];
      persistent = toupper(*argv[2]);

      if (persistent != 'P' && persistent != 'N') {
            ti->error = "Persistent flag is not P or N";
            r = -EINVAL;
            goto bad1;
      }

      s = kmalloc(sizeof(*s), GFP_KERNEL);
      if (s == NULL) {
            ti->error = "Cannot allocate snapshot context private "
                "structure";
            r = -ENOMEM;
            goto bad1;
      }

      r = dm_get_device(ti, origin_path, 0, ti->len, FMODE_READ, &s->origin);
      if (r) {
            ti->error = "Cannot get origin device";
            goto bad2;
      }

      r = dm_get_device(ti, cow_path, 0, 0,
                    FMODE_READ | FMODE_WRITE, &s->cow);
      if (r) {
            dm_put_device(ti, s->origin);
            ti->error = "Cannot get COW device";
            goto bad2;
      }

      r = set_chunk_size(s, argv[3], &ti->error);
      if (r)
            goto bad3;

      s->type = persistent;

      s->valid = 1;
      s->active = 0;
      s->last_percent = 0;
      init_rwsem(&s->lock);
      spin_lock_init(&s->pe_lock);
      s->table = ti->table;

      /* Allocate hash table for COW data */
      if (init_hash_tables(s)) {
            ti->error = "Unable to allocate hash table space";
            r = -ENOMEM;
            goto bad3;
      }

      s->store.snap = s;

      if (persistent == 'P')
            r = dm_create_persistent(&s->store);
      else
            r = dm_create_transient(&s->store);

      if (r) {
            ti->error = "Couldn't create exception store";
            r = -EINVAL;
            goto bad4;
      }

      r = kcopyd_client_create(SNAPSHOT_PAGES, &s->kcopyd_client);
      if (r) {
            ti->error = "Could not create kcopyd client";
            goto bad5;
      }

      /* Metadata must only be loaded into one table at once */
      r = s->store.read_metadata(&s->store);
      if (r < 0) {
            ti->error = "Failed to read snapshot metadata";
            goto bad6;
      } else if (r > 0) {
            s->valid = 0;
            DMWARN("Snapshot is marked invalid.");
      }

      bio_list_init(&s->queued_bios);
      INIT_WORK(&s->queued_bios_work, flush_queued_bios);

      /* Add snapshot to the list of snapshots for this origin */
      /* Exceptions aren't triggered till snapshot_resume() is called */
      if (register_snapshot(s)) {
            r = -EINVAL;
            ti->error = "Cannot register snapshot origin";
            goto bad6;
      }

      ti->private = s;
      ti->split_io = s->chunk_size;

      return 0;

 bad6:
      kcopyd_client_destroy(s->kcopyd_client);

 bad5:
      s->store.destroy(&s->store);

 bad4:
      exit_exception_table(&s->pending, pending_cache);
      exit_exception_table(&s->complete, exception_cache);

 bad3:
      dm_put_device(ti, s->cow);
      dm_put_device(ti, s->origin);

 bad2:
      kfree(s);

 bad1:
      return r;
}

static void __free_exceptions(struct dm_snapshot *s)
{
      kcopyd_client_destroy(s->kcopyd_client);
      s->kcopyd_client = NULL;

      exit_exception_table(&s->pending, pending_cache);
      exit_exception_table(&s->complete, exception_cache);

      s->store.destroy(&s->store);
}

static void snapshot_dtr(struct dm_target *ti)
{
      struct dm_snapshot *s = ti->private;

      flush_workqueue(ksnapd);

      /* Prevent further origin writes from using this snapshot. */
      /* After this returns there can be no new kcopyd jobs. */
      unregister_snapshot(s);

      __free_exceptions(s);

      dm_put_device(ti, s->origin);
      dm_put_device(ti, s->cow);

      kfree(s);
}

/*
 * Flush a list of buffers.
 */
static void flush_bios(struct bio *bio)
{
      struct bio *n;

      while (bio) {
            n = bio->bi_next;
            bio->bi_next = NULL;
            generic_make_request(bio);
            bio = n;
      }
}

static void flush_queued_bios(struct work_struct *work)
{
      struct dm_snapshot *s =
            container_of(work, struct dm_snapshot, queued_bios_work);
      struct bio *queued_bios;
      unsigned long flags;

      spin_lock_irqsave(&s->pe_lock, flags);
      queued_bios = bio_list_get(&s->queued_bios);
      spin_unlock_irqrestore(&s->pe_lock, flags);

      flush_bios(queued_bios);
}

/*
 * Error a list of buffers.
 */
static void error_bios(struct bio *bio)
{
      struct bio *n;

      while (bio) {
            n = bio->bi_next;
            bio->bi_next = NULL;
            bio_io_error(bio);
            bio = n;
      }
}

static void __invalidate_snapshot(struct dm_snapshot *s, int err)
{
      if (!s->valid)
            return;

      if (err == -EIO)
            DMERR("Invalidating snapshot: Error reading/writing.");
      else if (err == -ENOMEM)
            DMERR("Invalidating snapshot: Unable to allocate exception.");

      if (s->store.drop_snapshot)
            s->store.drop_snapshot(&s->store);

      s->valid = 0;

      dm_table_event(s->table);
}

static void get_pending_exception(struct dm_snap_pending_exception *pe)
{
      atomic_inc(&pe->ref_count);
}

static struct bio *put_pending_exception(struct dm_snap_pending_exception *pe)
{
      struct dm_snap_pending_exception *primary_pe;
      struct bio *origin_bios = NULL;

      primary_pe = pe->primary_pe;

      /*
       * If this pe is involved in a write to the origin and
       * it is the last sibling to complete then release
       * the bios for the original write to the origin.
       */
      if (primary_pe &&
          atomic_dec_and_test(&primary_pe->ref_count))
            origin_bios = bio_list_get(&primary_pe->origin_bios);

      /*
       * Free the pe if it's not linked to an origin write or if
       * it's not itself a primary pe.
       */
      if (!primary_pe || primary_pe != pe)
            free_pending_exception(pe);

      /*
       * Free the primary pe if nothing references it.
       */
      if (primary_pe && !atomic_read(&primary_pe->ref_count))
            free_pending_exception(primary_pe);

      return origin_bios;
}

static void pending_complete(struct dm_snap_pending_exception *pe, int success)
{
      struct dm_snap_exception *e;
      struct dm_snapshot *s = pe->snap;
      struct bio *origin_bios = NULL;
      struct bio *snapshot_bios = NULL;
      int error = 0;

      if (!success) {
            /* Read/write error - snapshot is unusable */
            down_write(&s->lock);
            __invalidate_snapshot(s, -EIO);
            error = 1;
            goto out;
      }

      e = alloc_exception();
      if (!e) {
            down_write(&s->lock);
            __invalidate_snapshot(s, -ENOMEM);
            error = 1;
            goto out;
      }
      *e = pe->e;

      down_write(&s->lock);
      if (!s->valid) {
            free_exception(e);
            error = 1;
            goto out;
      }

      /*
       * Add a proper exception, and remove the
       * in-flight exception from the list.
       */
      insert_exception(&s->complete, e);

 out:
      remove_exception(&pe->e);
      snapshot_bios = bio_list_get(&pe->snapshot_bios);
      origin_bios = put_pending_exception(pe);

      up_write(&s->lock);

      /* Submit any pending write bios */
      if (error)
            error_bios(snapshot_bios);
      else
            flush_bios(snapshot_bios);

      flush_bios(origin_bios);
}

static void commit_callback(void *context, int success)
{
      struct dm_snap_pending_exception *pe = context;

      pending_complete(pe, success);
}

/*
 * Called when the copy I/O has finished.  kcopyd actually runs
 * this code so don't block.
 */
static void copy_callback(int read_err, unsigned int write_err, void *context)
{
      struct dm_snap_pending_exception *pe = context;
      struct dm_snapshot *s = pe->snap;

      if (read_err || write_err)
            pending_complete(pe, 0);

      else
            /* Update the metadata if we are persistent */
            s->store.commit_exception(&s->store, &pe->e, commit_callback,
                                pe);
}

/*
 * Dispatches the copy operation to kcopyd.
 */
static void start_copy(struct dm_snap_pending_exception *pe)
{
      struct dm_snapshot *s = pe->snap;
      struct io_region src, dest;
      struct block_device *bdev = s->origin->bdev;
      sector_t dev_size;

      dev_size = get_dev_size(bdev);

      src.bdev = bdev;
      src.sector = chunk_to_sector(s, pe->e.old_chunk);
      src.count = min(s->chunk_size, dev_size - src.sector);

      dest.bdev = s->cow->bdev;
      dest.sector = chunk_to_sector(s, pe->e.new_chunk);
      dest.count = src.count;

      /* Hand over to kcopyd */
      kcopyd_copy(s->kcopyd_client,
                &src, 1, &dest, 0, copy_callback, pe);
}

/*
 * Looks to see if this snapshot already has a pending exception
 * for this chunk, otherwise it allocates a new one and inserts
 * it into the pending table.
 *
 * NOTE: a write lock must be held on snap->lock before calling
 * this.
 */
static struct dm_snap_pending_exception *
__find_pending_exception(struct dm_snapshot *s, struct bio *bio)
{
      struct dm_snap_exception *e;
      struct dm_snap_pending_exception *pe;
      chunk_t chunk = sector_to_chunk(s, bio->bi_sector);

      /*
       * Is there a pending exception for this already ?
       */
      e = lookup_exception(&s->pending, chunk);
      if (e) {
            /* cast the exception to a pending exception */
            pe = container_of(e, struct dm_snap_pending_exception, e);
            goto out;
      }

      /*
       * Create a new pending exception, we don't want
       * to hold the lock while we do this.
       */
      up_write(&s->lock);
      pe = alloc_pending_exception();
      down_write(&s->lock);

      if (!s->valid) {
            free_pending_exception(pe);
            return NULL;
      }

      e = lookup_exception(&s->pending, chunk);
      if (e) {
            free_pending_exception(pe);
            pe = container_of(e, struct dm_snap_pending_exception, e);
            goto out;
      }

      pe->e.old_chunk = chunk;
      bio_list_init(&pe->origin_bios);
      bio_list_init(&pe->snapshot_bios);
      pe->primary_pe = NULL;
      atomic_set(&pe->ref_count, 0);
      pe->snap = s;
      pe->started = 0;

      if (s->store.prepare_exception(&s->store, &pe->e)) {
            free_pending_exception(pe);
            return NULL;
      }

      get_pending_exception(pe);
      insert_exception(&s->pending, &pe->e);

 out:
      return pe;
}

static void remap_exception(struct dm_snapshot *s, struct dm_snap_exception *e,
                      struct bio *bio)
{
      bio->bi_bdev = s->cow->bdev;
      bio->bi_sector = chunk_to_sector(s, e->new_chunk) +
            (bio->bi_sector & s->chunk_mask);
}

static int snapshot_map(struct dm_target *ti, struct bio *bio,
                  union map_info *map_context)
{
      struct dm_snap_exception *e;
      struct dm_snapshot *s = ti->private;
      int r = DM_MAPIO_REMAPPED;
      chunk_t chunk;
      struct dm_snap_pending_exception *pe = NULL;

      chunk = sector_to_chunk(s, bio->bi_sector);

      /* Full snapshots are not usable */
      /* To get here the table must be live so s->active is always set. */
      if (!s->valid)
            return -EIO;

      /* FIXME: should only take write lock if we need
       * to copy an exception */
      down_write(&s->lock);

      if (!s->valid) {
            r = -EIO;
            goto out_unlock;
      }

      /* If the block is already remapped - use that, else remap it */
      e = lookup_exception(&s->complete, chunk);
      if (e) {
            remap_exception(s, e, bio);
            goto out_unlock;
      }

      /*
       * Write to snapshot - higher level takes care of RW/RO
       * flags so we should only get this if we are
       * writeable.
       */
      if (bio_rw(bio) == WRITE) {
            pe = __find_pending_exception(s, bio);
            if (!pe) {
                  __invalidate_snapshot(s, -ENOMEM);
                  r = -EIO;
                  goto out_unlock;
            }

            remap_exception(s, &pe->e, bio);
            bio_list_add(&pe->snapshot_bios, bio);

            r = DM_MAPIO_SUBMITTED;

            if (!pe->started) {
                  /* this is protected by snap->lock */
                  pe->started = 1;
                  up_write(&s->lock);
                  start_copy(pe);
                  goto out;
            }
      } else
            /*
             * FIXME: this read path scares me because we
             * always use the origin when we have a pending
             * exception.  However I can't think of a
             * situation where this is wrong - ejt.
             */
            bio->bi_bdev = s->origin->bdev;

 out_unlock:
      up_write(&s->lock);
 out:
      return r;
}

static void snapshot_resume(struct dm_target *ti)
{
      struct dm_snapshot *s = ti->private;

      down_write(&s->lock);
      s->active = 1;
      up_write(&s->lock);
}

static int snapshot_status(struct dm_target *ti, status_type_t type,
                     char *result, unsigned int maxlen)
{
      struct dm_snapshot *snap = ti->private;

      switch (type) {
      case STATUSTYPE_INFO:
            if (!snap->valid)
                  snprintf(result, maxlen, "Invalid");
            else {
                  if (snap->store.fraction_full) {
                        sector_t numerator, denominator;
                        snap->store.fraction_full(&snap->store,
                                            &numerator,
                                            &denominator);
                        snprintf(result, maxlen, "%llu/%llu",
                              (unsigned long long)numerator,
                              (unsigned long long)denominator);
                  }
                  else
                        snprintf(result, maxlen, "Unknown");
            }
            break;

      case STATUSTYPE_TABLE:
            /*
             * kdevname returns a static pointer so we need
             * to make private copies if the output is to
             * make sense.
             */
            snprintf(result, maxlen, "%s %s %c %llu",
                   snap->origin->name, snap->cow->name,
                   snap->type,
                   (unsigned long long)snap->chunk_size);
            break;
      }

      return 0;
}

/*-----------------------------------------------------------------
 * Origin methods
 *---------------------------------------------------------------*/
static int __origin_write(struct list_head *snapshots, struct bio *bio)
{
      int r = DM_MAPIO_REMAPPED, first = 0;
      struct dm_snapshot *snap;
      struct dm_snap_exception *e;
      struct dm_snap_pending_exception *pe, *next_pe, *primary_pe = NULL;
      chunk_t chunk;
      LIST_HEAD(pe_queue);

      /* Do all the snapshots on this origin */
      list_for_each_entry (snap, snapshots, list) {

            down_write(&snap->lock);

            /* Only deal with valid and active snapshots */
            if (!snap->valid || !snap->active)
                  goto next_snapshot;

            /* Nothing to do if writing beyond end of snapshot */
            if (bio->bi_sector >= dm_table_get_size(snap->table))
                  goto next_snapshot;

            /*
             * Remember, different snapshots can have
             * different chunk sizes.
             */
            chunk = sector_to_chunk(snap, bio->bi_sector);

            /*
             * Check exception table to see if block
             * is already remapped in this snapshot
             * and trigger an exception if not.
             *
             * ref_count is initialised to 1 so pending_complete()
             * won't destroy the primary_pe while we're inside this loop.
             */
            e = lookup_exception(&snap->complete, chunk);
            if (e)
                  goto next_snapshot;

            pe = __find_pending_exception(snap, bio);
            if (!pe) {
                  __invalidate_snapshot(snap, -ENOMEM);
                  goto next_snapshot;
            }

            if (!primary_pe) {
                  /*
                   * Either every pe here has same
                   * primary_pe or none has one yet.
                   */
                  if (pe->primary_pe)
                        primary_pe = pe->primary_pe;
                  else {
                        primary_pe = pe;
                        first = 1;
                  }

                  bio_list_add(&primary_pe->origin_bios, bio);

                  r = DM_MAPIO_SUBMITTED;
            }

            if (!pe->primary_pe) {
                  pe->primary_pe = primary_pe;
                  get_pending_exception(primary_pe);
            }

            if (!pe->started) {
                  pe->started = 1;
                  list_add_tail(&pe->list, &pe_queue);
            }

 next_snapshot:
            up_write(&snap->lock);
      }

      if (!primary_pe)
            return r;

      /*
       * If this is the first time we're processing this chunk and
       * ref_count is now 1 it means all the pending exceptions
       * got completed while we were in the loop above, so it falls to
       * us here to remove the primary_pe and submit any origin_bios.
       */

      if (first && atomic_dec_and_test(&primary_pe->ref_count)) {
            flush_bios(bio_list_get(&primary_pe->origin_bios));
            free_pending_exception(primary_pe);
            /* If we got here, pe_queue is necessarily empty. */
            return r;
      }

      /*
       * Now that we have a complete pe list we can start the copying.
       */
      list_for_each_entry_safe(pe, next_pe, &pe_queue, list)
            start_copy(pe);

      return r;
}

/*
 * Called on a write from the origin driver.
 */
static int do_origin(struct dm_dev *origin, struct bio *bio)
{
      struct origin *o;
      int r = DM_MAPIO_REMAPPED;

      down_read(&_origins_lock);
      o = __lookup_origin(origin->bdev);
      if (o)
            r = __origin_write(&o->snapshots, bio);
      up_read(&_origins_lock);

      return r;
}

/*
 * Origin: maps a linear range of a device, with hooks for snapshotting.
 */

/*
 * Construct an origin mapping: <dev_path>
 * The context for an origin is merely a 'struct dm_dev *'
 * pointing to the real device.
 */
static int origin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
      int r;
      struct dm_dev *dev;

      if (argc != 1) {
            ti->error = "origin: incorrect number of arguments";
            return -EINVAL;
      }

      r = dm_get_device(ti, argv[0], 0, ti->len,
                    dm_table_get_mode(ti->table), &dev);
      if (r) {
            ti->error = "Cannot get target device";
            return r;
      }

      ti->private = dev;
      return 0;
}

static void origin_dtr(struct dm_target *ti)
{
      struct dm_dev *dev = ti->private;
      dm_put_device(ti, dev);
}

static int origin_map(struct dm_target *ti, struct bio *bio,
                  union map_info *map_context)
{
      struct dm_dev *dev = ti->private;
      bio->bi_bdev = dev->bdev;

      /* Only tell snapshots if this is a write */
      return (bio_rw(bio) == WRITE) ? do_origin(dev, bio) : DM_MAPIO_REMAPPED;
}

#define min_not_zero(l, r) (l == 0) ? r : ((r == 0) ? l : min(l, r))

/*
 * Set the target "split_io" field to the minimum of all the snapshots'
 * chunk sizes.
 */
static void origin_resume(struct dm_target *ti)
{
      struct dm_dev *dev = ti->private;
      struct dm_snapshot *snap;
      struct origin *o;
      chunk_t chunk_size = 0;

      down_read(&_origins_lock);
      o = __lookup_origin(dev->bdev);
      if (o)
            list_for_each_entry (snap, &o->snapshots, list)
                  chunk_size = min_not_zero(chunk_size, snap->chunk_size);
      up_read(&_origins_lock);

      ti->split_io = chunk_size;
}

static int origin_status(struct dm_target *ti, status_type_t type, char *result,
                   unsigned int maxlen)
{
      struct dm_dev *dev = ti->private;

      switch (type) {
      case STATUSTYPE_INFO:
            result[0] = '\0';
            break;

      case STATUSTYPE_TABLE:
            snprintf(result, maxlen, "%s", dev->name);
            break;
      }

      return 0;
}

static struct target_type origin_target = {
      .name    = "snapshot-origin",
      .version = {1, 5, 0},
      .module  = THIS_MODULE,
      .ctr     = origin_ctr,
      .dtr     = origin_dtr,
      .map     = origin_map,
      .resume  = origin_resume,
      .status  = origin_status,
};

static struct target_type snapshot_target = {
      .name    = "snapshot",
      .version = {1, 5, 0},
      .module  = THIS_MODULE,
      .ctr     = snapshot_ctr,
      .dtr     = snapshot_dtr,
      .map     = snapshot_map,
      .resume  = snapshot_resume,
      .status  = snapshot_status,
};

static int __init dm_snapshot_init(void)
{
      int r;

      r = dm_register_target(&snapshot_target);
      if (r) {
            DMERR("snapshot target register failed %d", r);
            return r;
      }

      r = dm_register_target(&origin_target);
      if (r < 0) {
            DMERR("Origin target register failed %d", r);
            goto bad1;
      }

      r = init_origin_hash();
      if (r) {
            DMERR("init_origin_hash failed.");
            goto bad2;
      }

      exception_cache = KMEM_CACHE(dm_snap_exception, 0);
      if (!exception_cache) {
            DMERR("Couldn't create exception cache.");
            r = -ENOMEM;
            goto bad3;
      }

      pending_cache = KMEM_CACHE(dm_snap_pending_exception, 0);
      if (!pending_cache) {
            DMERR("Couldn't create pending cache.");
            r = -ENOMEM;
            goto bad4;
      }

      pending_pool = mempool_create_slab_pool(128, pending_cache);
      if (!pending_pool) {
            DMERR("Couldn't create pending pool.");
            r = -ENOMEM;
            goto bad5;
      }

      ksnapd = create_singlethread_workqueue("ksnapd");
      if (!ksnapd) {
            DMERR("Failed to create ksnapd workqueue.");
            r = -ENOMEM;
            goto bad6;
      }

      return 0;

      bad6:
      mempool_destroy(pending_pool);
      bad5:
      kmem_cache_destroy(pending_cache);
      bad4:
      kmem_cache_destroy(exception_cache);
      bad3:
      exit_origin_hash();
      bad2:
      dm_unregister_target(&origin_target);
      bad1:
      dm_unregister_target(&snapshot_target);
      return r;
}

static void __exit dm_snapshot_exit(void)
{
      int r;

      destroy_workqueue(ksnapd);

      r = dm_unregister_target(&snapshot_target);
      if (r)
            DMERR("snapshot unregister failed %d", r);

      r = dm_unregister_target(&origin_target);
      if (r)
            DMERR("origin unregister failed %d", r);

      exit_origin_hash();
      mempool_destroy(pending_pool);
      kmem_cache_destroy(pending_cache);
      kmem_cache_destroy(exception_cache);
}

/* Module hooks */
module_init(dm_snapshot_init);
module_exit(dm_snapshot_exit);

MODULE_DESCRIPTION(DM_NAME " snapshot target");
MODULE_AUTHOR("Joe Thornber");
MODULE_LICENSE("GPL");

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