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relay.c

/*
 * Public API and common code for kernel->userspace relay file support.
 *
 * See Documentation/filesystems/relay.txt for an overview.
 *
 * Copyright (C) 2002-2005 - Tom Zanussi (zanussi@us.ibm.com), IBM Corp
 * Copyright (C) 1999-2005 - Karim Yaghmour (karim@opersys.com)
 *
 * Moved to kernel/relay.c by Paul Mundt, 2006.
 * November 2006 - CPU hotplug support by Mathieu Desnoyers
 *    (mathieu.desnoyers@polymtl.ca)
 *
 * This file is released under the GPL.
 */
#include <linux/errno.h>
#include <linux/stddef.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/string.h>
#include <linux/relay.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>
#include <linux/cpu.h>
#include <linux/splice.h>

/* list of open channels, for cpu hotplug */
static DEFINE_MUTEX(relay_channels_mutex);
static LIST_HEAD(relay_channels);

/*
 * close() vm_op implementation for relay file mapping.
 */
static void relay_file_mmap_close(struct vm_area_struct *vma)
{
      struct rchan_buf *buf = vma->vm_private_data;
      buf->chan->cb->buf_unmapped(buf, vma->vm_file);
}

/*
 * nopage() vm_op implementation for relay file mapping.
 */
static struct page *relay_buf_nopage(struct vm_area_struct *vma,
                             unsigned long address,
                             int *type)
{
      struct page *page;
      struct rchan_buf *buf = vma->vm_private_data;
      unsigned long offset = address - vma->vm_start;

      if (address > vma->vm_end)
            return NOPAGE_SIGBUS; /* Disallow mremap */
      if (!buf)
            return NOPAGE_OOM;

      page = vmalloc_to_page(buf->start + offset);
      if (!page)
            return NOPAGE_OOM;
      get_page(page);

      if (type)
            *type = VM_FAULT_MINOR;

      return page;
}

/*
 * vm_ops for relay file mappings.
 */
static struct vm_operations_struct relay_file_mmap_ops = {
      .nopage = relay_buf_nopage,
      .close = relay_file_mmap_close,
};

/**
 *    relay_mmap_buf: - mmap channel buffer to process address space
 *    @buf: relay channel buffer
 *    @vma: vm_area_struct describing memory to be mapped
 *
 *    Returns 0 if ok, negative on error
 *
 *    Caller should already have grabbed mmap_sem.
 */
static int relay_mmap_buf(struct rchan_buf *buf, struct vm_area_struct *vma)
{
      unsigned long length = vma->vm_end - vma->vm_start;
      struct file *filp = vma->vm_file;

      if (!buf)
            return -EBADF;

      if (length != (unsigned long)buf->chan->alloc_size)
            return -EINVAL;

      vma->vm_ops = &relay_file_mmap_ops;
      vma->vm_flags |= VM_DONTEXPAND;
      vma->vm_private_data = buf;
      buf->chan->cb->buf_mapped(buf, filp);

      return 0;
}

/**
 *    relay_alloc_buf - allocate a channel buffer
 *    @buf: the buffer struct
 *    @size: total size of the buffer
 *
 *    Returns a pointer to the resulting buffer, %NULL if unsuccessful. The
 *    passed in size will get page aligned, if it isn't already.
 */
static void *relay_alloc_buf(struct rchan_buf *buf, size_t *size)
{
      void *mem;
      unsigned int i, j, n_pages;

      *size = PAGE_ALIGN(*size);
      n_pages = *size >> PAGE_SHIFT;

      buf->page_array = kcalloc(n_pages, sizeof(struct page *), GFP_KERNEL);
      if (!buf->page_array)
            return NULL;

      for (i = 0; i < n_pages; i++) {
            buf->page_array[i] = alloc_page(GFP_KERNEL);
            if (unlikely(!buf->page_array[i]))
                  goto depopulate;
            set_page_private(buf->page_array[i], (unsigned long)buf);
      }
      mem = vmap(buf->page_array, n_pages, VM_MAP, PAGE_KERNEL);
      if (!mem)
            goto depopulate;

      memset(mem, 0, *size);
      buf->page_count = n_pages;
      return mem;

depopulate:
      for (j = 0; j < i; j++)
            __free_page(buf->page_array[j]);
      kfree(buf->page_array);
      return NULL;
}

/**
 *    relay_create_buf - allocate and initialize a channel buffer
 *    @chan: the relay channel
 *
 *    Returns channel buffer if successful, %NULL otherwise.
 */
static struct rchan_buf *relay_create_buf(struct rchan *chan)
{
      struct rchan_buf *buf = kzalloc(sizeof(struct rchan_buf), GFP_KERNEL);
      if (!buf)
            return NULL;

      buf->padding = kmalloc(chan->n_subbufs * sizeof(size_t *), GFP_KERNEL);
      if (!buf->padding)
            goto free_buf;

      buf->start = relay_alloc_buf(buf, &chan->alloc_size);
      if (!buf->start)
            goto free_buf;

      buf->chan = chan;
      kref_get(&buf->chan->kref);
      return buf;

free_buf:
      kfree(buf->padding);
      kfree(buf);
      return NULL;
}

/**
 *    relay_destroy_channel - free the channel struct
 *    @kref: target kernel reference that contains the relay channel
 *
 *    Should only be called from kref_put().
 */
static void relay_destroy_channel(struct kref *kref)
{
      struct rchan *chan = container_of(kref, struct rchan, kref);
      kfree(chan);
}

/**
 *    relay_destroy_buf - destroy an rchan_buf struct and associated buffer
 *    @buf: the buffer struct
 */
static void relay_destroy_buf(struct rchan_buf *buf)
{
      struct rchan *chan = buf->chan;
      unsigned int i;

      if (likely(buf->start)) {
            vunmap(buf->start);
            for (i = 0; i < buf->page_count; i++)
                  __free_page(buf->page_array[i]);
            kfree(buf->page_array);
      }
      chan->buf[buf->cpu] = NULL;
      kfree(buf->padding);
      kfree(buf);
      kref_put(&chan->kref, relay_destroy_channel);
}

/**
 *    relay_remove_buf - remove a channel buffer
 *    @kref: target kernel reference that contains the relay buffer
 *
 *    Removes the file from the fileystem, which also frees the
 *    rchan_buf_struct and the channel buffer.  Should only be called from
 *    kref_put().
 */
static void relay_remove_buf(struct kref *kref)
{
      struct rchan_buf *buf = container_of(kref, struct rchan_buf, kref);
      buf->chan->cb->remove_buf_file(buf->dentry);
      relay_destroy_buf(buf);
}

/**
 *    relay_buf_empty - boolean, is the channel buffer empty?
 *    @buf: channel buffer
 *
 *    Returns 1 if the buffer is empty, 0 otherwise.
 */
static int relay_buf_empty(struct rchan_buf *buf)
{
      return (buf->subbufs_produced - buf->subbufs_consumed) ? 0 : 1;
}

/**
 *    relay_buf_full - boolean, is the channel buffer full?
 *    @buf: channel buffer
 *
 *    Returns 1 if the buffer is full, 0 otherwise.
 */
int relay_buf_full(struct rchan_buf *buf)
{
      size_t ready = buf->subbufs_produced - buf->subbufs_consumed;
      return (ready >= buf->chan->n_subbufs) ? 1 : 0;
}
EXPORT_SYMBOL_GPL(relay_buf_full);

/*
 * High-level relay kernel API and associated functions.
 */

/*
 * rchan_callback implementations defining default channel behavior.  Used
 * in place of corresponding NULL values in client callback struct.
 */

/*
 * subbuf_start() default callback.  Does nothing.
 */
static int subbuf_start_default_callback (struct rchan_buf *buf,
                                void *subbuf,
                                void *prev_subbuf,
                                size_t prev_padding)
{
      if (relay_buf_full(buf))
            return 0;

      return 1;
}

/*
 * buf_mapped() default callback.  Does nothing.
 */
static void buf_mapped_default_callback(struct rchan_buf *buf,
                              struct file *filp)
{
}

/*
 * buf_unmapped() default callback.  Does nothing.
 */
static void buf_unmapped_default_callback(struct rchan_buf *buf,
                                struct file *filp)
{
}

/*
 * create_buf_file_create() default callback.  Does nothing.
 */
static struct dentry *create_buf_file_default_callback(const char *filename,
                                           struct dentry *parent,
                                           int mode,
                                           struct rchan_buf *buf,
                                           int *is_global)
{
      return NULL;
}

/*
 * remove_buf_file() default callback.  Does nothing.
 */
static int remove_buf_file_default_callback(struct dentry *dentry)
{
      return -EINVAL;
}

/* relay channel default callbacks */
static struct rchan_callbacks default_channel_callbacks = {
      .subbuf_start = subbuf_start_default_callback,
      .buf_mapped = buf_mapped_default_callback,
      .buf_unmapped = buf_unmapped_default_callback,
      .create_buf_file = create_buf_file_default_callback,
      .remove_buf_file = remove_buf_file_default_callback,
};

/**
 *    wakeup_readers - wake up readers waiting on a channel
 *    @data: contains the channel buffer
 *
 *    This is the timer function used to defer reader waking.
 */
static void wakeup_readers(unsigned long data)
{
      struct rchan_buf *buf = (struct rchan_buf *)data;
      wake_up_interruptible(&buf->read_wait);
}

/**
 *    __relay_reset - reset a channel buffer
 *    @buf: the channel buffer
 *    @init: 1 if this is a first-time initialization
 *
 *    See relay_reset() for description of effect.
 */
static void __relay_reset(struct rchan_buf *buf, unsigned int init)
{
      size_t i;

      if (init) {
            init_waitqueue_head(&buf->read_wait);
            kref_init(&buf->kref);
            setup_timer(&buf->timer, wakeup_readers, (unsigned long)buf);
      } else
            del_timer_sync(&buf->timer);

      buf->subbufs_produced = 0;
      buf->subbufs_consumed = 0;
      buf->bytes_consumed = 0;
      buf->finalized = 0;
      buf->data = buf->start;
      buf->offset = 0;

      for (i = 0; i < buf->chan->n_subbufs; i++)
            buf->padding[i] = 0;

      buf->chan->cb->subbuf_start(buf, buf->data, NULL, 0);
}

/**
 *    relay_reset - reset the channel
 *    @chan: the channel
 *
 *    This has the effect of erasing all data from all channel buffers
 *    and restarting the channel in its initial state.  The buffers
 *    are not freed, so any mappings are still in effect.
 *
 *    NOTE. Care should be taken that the channel isn't actually
 *    being used by anything when this call is made.
 */
void relay_reset(struct rchan *chan)
{
      unsigned int i;

      if (!chan)
            return;

      if (chan->is_global && chan->buf[0]) {
            __relay_reset(chan->buf[0], 0);
            return;
      }

      mutex_lock(&relay_channels_mutex);
      for_each_online_cpu(i)
            if (chan->buf[i])
                  __relay_reset(chan->buf[i], 0);
      mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_reset);

/*
 *    relay_open_buf - create a new relay channel buffer
 *
 *    used by relay_open() and CPU hotplug.
 */
static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
{
      struct rchan_buf *buf = NULL;
      struct dentry *dentry;
      char *tmpname;

      if (chan->is_global)
            return chan->buf[0];

      tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
      if (!tmpname)
            goto end;
      snprintf(tmpname, NAME_MAX, "%s%d", chan->base_filename, cpu);

      buf = relay_create_buf(chan);
      if (!buf)
            goto free_name;

      buf->cpu = cpu;
      __relay_reset(buf, 1);

      /* Create file in fs */
      dentry = chan->cb->create_buf_file(tmpname, chan->parent, S_IRUSR,
                                 buf, &chan->is_global);
      if (!dentry)
            goto free_buf;

      buf->dentry = dentry;

      if(chan->is_global) {
            chan->buf[0] = buf;
            buf->cpu = 0;
      }

      goto free_name;

free_buf:
      relay_destroy_buf(buf);
      buf = NULL;
free_name:
      kfree(tmpname);
end:
      return buf;
}

/**
 *    relay_close_buf - close a channel buffer
 *    @buf: channel buffer
 *
 *    Marks the buffer finalized and restores the default callbacks.
 *    The channel buffer and channel buffer data structure are then freed
 *    automatically when the last reference is given up.
 */
static void relay_close_buf(struct rchan_buf *buf)
{
      buf->finalized = 1;
      del_timer_sync(&buf->timer);
      kref_put(&buf->kref, relay_remove_buf);
}

static void setup_callbacks(struct rchan *chan,
                           struct rchan_callbacks *cb)
{
      if (!cb) {
            chan->cb = &default_channel_callbacks;
            return;
      }

      if (!cb->subbuf_start)
            cb->subbuf_start = subbuf_start_default_callback;
      if (!cb->buf_mapped)
            cb->buf_mapped = buf_mapped_default_callback;
      if (!cb->buf_unmapped)
            cb->buf_unmapped = buf_unmapped_default_callback;
      if (!cb->create_buf_file)
            cb->create_buf_file = create_buf_file_default_callback;
      if (!cb->remove_buf_file)
            cb->remove_buf_file = remove_buf_file_default_callback;
      chan->cb = cb;
}

/**
 *    relay_hotcpu_callback - CPU hotplug callback
 *    @nb: notifier block
 *    @action: hotplug action to take
 *    @hcpu: CPU number
 *
 *    Returns the success/failure of the operation. (%NOTIFY_OK, %NOTIFY_BAD)
 */
static int __cpuinit relay_hotcpu_callback(struct notifier_block *nb,
                        unsigned long action,
                        void *hcpu)
{
      unsigned int hotcpu = (unsigned long)hcpu;
      struct rchan *chan;

      switch(action) {
      case CPU_UP_PREPARE:
      case CPU_UP_PREPARE_FROZEN:
            mutex_lock(&relay_channels_mutex);
            list_for_each_entry(chan, &relay_channels, list) {
                  if (chan->buf[hotcpu])
                        continue;
                  chan->buf[hotcpu] = relay_open_buf(chan, hotcpu);
                  if(!chan->buf[hotcpu]) {
                        printk(KERN_ERR
                              "relay_hotcpu_callback: cpu %d buffer "
                              "creation failed\n", hotcpu);
                        mutex_unlock(&relay_channels_mutex);
                        return NOTIFY_BAD;
                  }
            }
            mutex_unlock(&relay_channels_mutex);
            break;
      case CPU_DEAD:
      case CPU_DEAD_FROZEN:
            /* No need to flush the cpu : will be flushed upon
             * final relay_flush() call. */
            break;
      }
      return NOTIFY_OK;
}

/**
 *    relay_open - create a new relay channel
 *    @base_filename: base name of files to create
 *    @parent: dentry of parent directory, %NULL for root directory
 *    @subbuf_size: size of sub-buffers
 *    @n_subbufs: number of sub-buffers
 *    @cb: client callback functions
 *    @private_data: user-defined data
 *
 *    Returns channel pointer if successful, %NULL otherwise.
 *
 *    Creates a channel buffer for each cpu using the sizes and
 *    attributes specified.  The created channel buffer files
 *    will be named base_filename0...base_filenameN-1.  File
 *    permissions will be %S_IRUSR.
 */
struct rchan *relay_open(const char *base_filename,
                   struct dentry *parent,
                   size_t subbuf_size,
                   size_t n_subbufs,
                   struct rchan_callbacks *cb,
                   void *private_data)
{
      unsigned int i;
      struct rchan *chan;
      if (!base_filename)
            return NULL;

      if (!(subbuf_size && n_subbufs))
            return NULL;

      chan = kzalloc(sizeof(struct rchan), GFP_KERNEL);
      if (!chan)
            return NULL;

      chan->version = RELAYFS_CHANNEL_VERSION;
      chan->n_subbufs = n_subbufs;
      chan->subbuf_size = subbuf_size;
      chan->alloc_size = FIX_SIZE(subbuf_size * n_subbufs);
      chan->parent = parent;
      chan->private_data = private_data;
      strlcpy(chan->base_filename, base_filename, NAME_MAX);
      setup_callbacks(chan, cb);
      kref_init(&chan->kref);

      mutex_lock(&relay_channels_mutex);
      for_each_online_cpu(i) {
            chan->buf[i] = relay_open_buf(chan, i);
            if (!chan->buf[i])
                  goto free_bufs;
      }
      list_add(&chan->list, &relay_channels);
      mutex_unlock(&relay_channels_mutex);

      return chan;

free_bufs:
      for_each_online_cpu(i) {
            if (!chan->buf[i])
                  break;
            relay_close_buf(chan->buf[i]);
      }

      kref_put(&chan->kref, relay_destroy_channel);
      mutex_unlock(&relay_channels_mutex);
      return NULL;
}
EXPORT_SYMBOL_GPL(relay_open);

/**
 *    relay_switch_subbuf - switch to a new sub-buffer
 *    @buf: channel buffer
 *    @length: size of current event
 *
 *    Returns either the length passed in or 0 if full.
 *
 *    Performs sub-buffer-switch tasks such as invoking callbacks,
 *    updating padding counts, waking up readers, etc.
 */
size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
{
      void *old, *new;
      size_t old_subbuf, new_subbuf;

      if (unlikely(length > buf->chan->subbuf_size))
            goto toobig;

      if (buf->offset != buf->chan->subbuf_size + 1) {
            buf->prev_padding = buf->chan->subbuf_size - buf->offset;
            old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
            buf->padding[old_subbuf] = buf->prev_padding;
            buf->subbufs_produced++;
            buf->dentry->d_inode->i_size += buf->chan->subbuf_size -
                  buf->padding[old_subbuf];
            smp_mb();
            if (waitqueue_active(&buf->read_wait))
                  /*
                   * Calling wake_up_interruptible() from here
                   * will deadlock if we happen to be logging
                   * from the scheduler (trying to re-grab
                   * rq->lock), so defer it.
                   */
                  __mod_timer(&buf->timer, jiffies + 1);
      }

      old = buf->data;
      new_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
      new = buf->start + new_subbuf * buf->chan->subbuf_size;
      buf->offset = 0;
      if (!buf->chan->cb->subbuf_start(buf, new, old, buf->prev_padding)) {
            buf->offset = buf->chan->subbuf_size + 1;
            return 0;
      }
      buf->data = new;
      buf->padding[new_subbuf] = 0;

      if (unlikely(length + buf->offset > buf->chan->subbuf_size))
            goto toobig;

      return length;

toobig:
      buf->chan->last_toobig = length;
      return 0;
}
EXPORT_SYMBOL_GPL(relay_switch_subbuf);

/**
 *    relay_subbufs_consumed - update the buffer's sub-buffers-consumed count
 *    @chan: the channel
 *    @cpu: the cpu associated with the channel buffer to update
 *    @subbufs_consumed: number of sub-buffers to add to current buf's count
 *
 *    Adds to the channel buffer's consumed sub-buffer count.
 *    subbufs_consumed should be the number of sub-buffers newly consumed,
 *    not the total consumed.
 *
 *    NOTE. Kernel clients don't need to call this function if the channel
 *    mode is 'overwrite'.
 */
void relay_subbufs_consumed(struct rchan *chan,
                      unsigned int cpu,
                      size_t subbufs_consumed)
{
      struct rchan_buf *buf;

      if (!chan)
            return;

      if (cpu >= NR_CPUS || !chan->buf[cpu])
            return;

      buf = chan->buf[cpu];
      buf->subbufs_consumed += subbufs_consumed;
      if (buf->subbufs_consumed > buf->subbufs_produced)
            buf->subbufs_consumed = buf->subbufs_produced;
}
EXPORT_SYMBOL_GPL(relay_subbufs_consumed);

/**
 *    relay_close - close the channel
 *    @chan: the channel
 *
 *    Closes all channel buffers and frees the channel.
 */
void relay_close(struct rchan *chan)
{
      unsigned int i;

      if (!chan)
            return;

      mutex_lock(&relay_channels_mutex);
      if (chan->is_global && chan->buf[0])
            relay_close_buf(chan->buf[0]);
      else
            for_each_possible_cpu(i)
                  if (chan->buf[i])
                        relay_close_buf(chan->buf[i]);

      if (chan->last_toobig)
            printk(KERN_WARNING "relay: one or more items not logged "
                   "[item size (%Zd) > sub-buffer size (%Zd)]\n",
                   chan->last_toobig, chan->subbuf_size);

      list_del(&chan->list);
      kref_put(&chan->kref, relay_destroy_channel);
      mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_close);

/**
 *    relay_flush - close the channel
 *    @chan: the channel
 *
 *    Flushes all channel buffers, i.e. forces buffer switch.
 */
void relay_flush(struct rchan *chan)
{
      unsigned int i;

      if (!chan)
            return;

      if (chan->is_global && chan->buf[0]) {
            relay_switch_subbuf(chan->buf[0], 0);
            return;
      }

      mutex_lock(&relay_channels_mutex);
      for_each_possible_cpu(i)
            if (chan->buf[i])
                  relay_switch_subbuf(chan->buf[i], 0);
      mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_flush);

/**
 *    relay_file_open - open file op for relay files
 *    @inode: the inode
 *    @filp: the file
 *
 *    Increments the channel buffer refcount.
 */
static int relay_file_open(struct inode *inode, struct file *filp)
{
      struct rchan_buf *buf = inode->i_private;
      kref_get(&buf->kref);
      filp->private_data = buf;

      return 0;
}

/**
 *    relay_file_mmap - mmap file op for relay files
 *    @filp: the file
 *    @vma: the vma describing what to map
 *
 *    Calls upon relay_mmap_buf() to map the file into user space.
 */
static int relay_file_mmap(struct file *filp, struct vm_area_struct *vma)
{
      struct rchan_buf *buf = filp->private_data;
      return relay_mmap_buf(buf, vma);
}

/**
 *    relay_file_poll - poll file op for relay files
 *    @filp: the file
 *    @wait: poll table
 *
 *    Poll implemention.
 */
static unsigned int relay_file_poll(struct file *filp, poll_table *wait)
{
      unsigned int mask = 0;
      struct rchan_buf *buf = filp->private_data;

      if (buf->finalized)
            return POLLERR;

      if (filp->f_mode & FMODE_READ) {
            poll_wait(filp, &buf->read_wait, wait);
            if (!relay_buf_empty(buf))
                  mask |= POLLIN | POLLRDNORM;
      }

      return mask;
}

/**
 *    relay_file_release - release file op for relay files
 *    @inode: the inode
 *    @filp: the file
 *
 *    Decrements the channel refcount, as the filesystem is
 *    no longer using it.
 */
static int relay_file_release(struct inode *inode, struct file *filp)
{
      struct rchan_buf *buf = filp->private_data;
      kref_put(&buf->kref, relay_remove_buf);

      return 0;
}

/*
 *    relay_file_read_consume - update the consumed count for the buffer
 */
static void relay_file_read_consume(struct rchan_buf *buf,
                            size_t read_pos,
                            size_t bytes_consumed)
{
      size_t subbuf_size = buf->chan->subbuf_size;
      size_t n_subbufs = buf->chan->n_subbufs;
      size_t read_subbuf;

      if (buf->bytes_consumed + bytes_consumed > subbuf_size) {
            relay_subbufs_consumed(buf->chan, buf->cpu, 1);
            buf->bytes_consumed = 0;
      }

      buf->bytes_consumed += bytes_consumed;
      if (!read_pos)
            read_subbuf = buf->subbufs_consumed % n_subbufs;
      else
            read_subbuf = read_pos / buf->chan->subbuf_size;
      if (buf->bytes_consumed + buf->padding[read_subbuf] == subbuf_size) {
            if ((read_subbuf == buf->subbufs_produced % n_subbufs) &&
                (buf->offset == subbuf_size))
                  return;
            relay_subbufs_consumed(buf->chan, buf->cpu, 1);
            buf->bytes_consumed = 0;
      }
}

/*
 *    relay_file_read_avail - boolean, are there unconsumed bytes available?
 */
static int relay_file_read_avail(struct rchan_buf *buf, size_t read_pos)
{
      size_t subbuf_size = buf->chan->subbuf_size;
      size_t n_subbufs = buf->chan->n_subbufs;
      size_t produced = buf->subbufs_produced;
      size_t consumed = buf->subbufs_consumed;

      relay_file_read_consume(buf, read_pos, 0);

      if (unlikely(buf->offset > subbuf_size)) {
            if (produced == consumed)
                  return 0;
            return 1;
      }

      if (unlikely(produced - consumed >= n_subbufs)) {
            consumed = produced - n_subbufs + 1;
            buf->subbufs_consumed = consumed;
            buf->bytes_consumed = 0;
      }

      produced = (produced % n_subbufs) * subbuf_size + buf->offset;
      consumed = (consumed % n_subbufs) * subbuf_size + buf->bytes_consumed;

      if (consumed > produced)
            produced += n_subbufs * subbuf_size;

      if (consumed == produced)
            return 0;

      return 1;
}

/**
 *    relay_file_read_subbuf_avail - return bytes available in sub-buffer
 *    @read_pos: file read position
 *    @buf: relay channel buffer
 */
static size_t relay_file_read_subbuf_avail(size_t read_pos,
                                 struct rchan_buf *buf)
{
      size_t padding, avail = 0;
      size_t read_subbuf, read_offset, write_subbuf, write_offset;
      size_t subbuf_size = buf->chan->subbuf_size;

      write_subbuf = (buf->data - buf->start) / subbuf_size;
      write_offset = buf->offset > subbuf_size ? subbuf_size : buf->offset;
      read_subbuf = read_pos / subbuf_size;
      read_offset = read_pos % subbuf_size;
      padding = buf->padding[read_subbuf];

      if (read_subbuf == write_subbuf) {
            if (read_offset + padding < write_offset)
                  avail = write_offset - (read_offset + padding);
      } else
            avail = (subbuf_size - padding) - read_offset;

      return avail;
}

/**
 *    relay_file_read_start_pos - find the first available byte to read
 *    @read_pos: file read position
 *    @buf: relay channel buffer
 *
 *    If the @read_pos is in the middle of padding, return the
 *    position of the first actually available byte, otherwise
 *    return the original value.
 */
static size_t relay_file_read_start_pos(size_t read_pos,
                              struct rchan_buf *buf)
{
      size_t read_subbuf, padding, padding_start, padding_end;
      size_t subbuf_size = buf->chan->subbuf_size;
      size_t n_subbufs = buf->chan->n_subbufs;
      size_t consumed = buf->subbufs_consumed % n_subbufs;

      if (!read_pos)
            read_pos = consumed * subbuf_size + buf->bytes_consumed;
      read_subbuf = read_pos / subbuf_size;
      padding = buf->padding[read_subbuf];
      padding_start = (read_subbuf + 1) * subbuf_size - padding;
      padding_end = (read_subbuf + 1) * subbuf_size;
      if (read_pos >= padding_start && read_pos < padding_end) {
            read_subbuf = (read_subbuf + 1) % n_subbufs;
            read_pos = read_subbuf * subbuf_size;
      }

      return read_pos;
}

/**
 *    relay_file_read_end_pos - return the new read position
 *    @read_pos: file read position
 *    @buf: relay channel buffer
 *    @count: number of bytes to be read
 */
static size_t relay_file_read_end_pos(struct rchan_buf *buf,
                              size_t read_pos,
                              size_t count)
{
      size_t read_subbuf, padding, end_pos;
      size_t subbuf_size = buf->chan->subbuf_size;
      size_t n_subbufs = buf->chan->n_subbufs;

      read_subbuf = read_pos / subbuf_size;
      padding = buf->padding[read_subbuf];
      if (read_pos % subbuf_size + count + padding == subbuf_size)
            end_pos = (read_subbuf + 1) * subbuf_size;
      else
            end_pos = read_pos + count;
      if (end_pos >= subbuf_size * n_subbufs)
            end_pos = 0;

      return end_pos;
}

/*
 *    subbuf_read_actor - read up to one subbuf's worth of data
 */
static int subbuf_read_actor(size_t read_start,
                       struct rchan_buf *buf,
                       size_t avail,
                       read_descriptor_t *desc,
                       read_actor_t actor)
{
      void *from;
      int ret = 0;

      from = buf->start + read_start;
      ret = avail;
      if (copy_to_user(desc->arg.buf, from, avail)) {
            desc->error = -EFAULT;
            ret = 0;
      }
      desc->arg.data += ret;
      desc->written += ret;
      desc->count -= ret;

      return ret;
}

typedef int (*subbuf_actor_t) (size_t read_start,
                         struct rchan_buf *buf,
                         size_t avail,
                         read_descriptor_t *desc,
                         read_actor_t actor);

/*
 *    relay_file_read_subbufs - read count bytes, bridging subbuf boundaries
 */
static ssize_t relay_file_read_subbufs(struct file *filp, loff_t *ppos,
                              subbuf_actor_t subbuf_actor,
                              read_actor_t actor,
                              read_descriptor_t *desc)
{
      struct rchan_buf *buf = filp->private_data;
      size_t read_start, avail;
      int ret;

      if (!desc->count)
            return 0;

      mutex_lock(&filp->f_path.dentry->d_inode->i_mutex);
      do {
            if (!relay_file_read_avail(buf, *ppos))
                  break;

            read_start = relay_file_read_start_pos(*ppos, buf);
            avail = relay_file_read_subbuf_avail(read_start, buf);
            if (!avail)
                  break;

            avail = min(desc->count, avail);
            ret = subbuf_actor(read_start, buf, avail, desc, actor);
            if (desc->error < 0)
                  break;

            if (ret) {
                  relay_file_read_consume(buf, read_start, ret);
                  *ppos = relay_file_read_end_pos(buf, read_start, ret);
            }
      } while (desc->count && ret);
      mutex_unlock(&filp->f_path.dentry->d_inode->i_mutex);

      return desc->written;
}

static ssize_t relay_file_read(struct file *filp,
                         char __user *buffer,
                         size_t count,
                         loff_t *ppos)
{
      read_descriptor_t desc;
      desc.written = 0;
      desc.count = count;
      desc.arg.buf = buffer;
      desc.error = 0;
      return relay_file_read_subbufs(filp, ppos, subbuf_read_actor,
                               NULL, &desc);
}

static void relay_consume_bytes(struct rchan_buf *rbuf, int bytes_consumed)
{
      rbuf->bytes_consumed += bytes_consumed;

      if (rbuf->bytes_consumed >= rbuf->chan->subbuf_size) {
            relay_subbufs_consumed(rbuf->chan, rbuf->cpu, 1);
            rbuf->bytes_consumed %= rbuf->chan->subbuf_size;
      }
}

static void relay_pipe_buf_release(struct pipe_inode_info *pipe,
                           struct pipe_buffer *buf)
{
      struct rchan_buf *rbuf;

      rbuf = (struct rchan_buf *)page_private(buf->page);
      relay_consume_bytes(rbuf, buf->private);
}

static struct pipe_buf_operations relay_pipe_buf_ops = {
      .can_merge = 0,
      .map = generic_pipe_buf_map,
      .unmap = generic_pipe_buf_unmap,
      .confirm = generic_pipe_buf_confirm,
      .release = relay_pipe_buf_release,
      .steal = generic_pipe_buf_steal,
      .get = generic_pipe_buf_get,
};

/*
 *    subbuf_splice_actor - splice up to one subbuf's worth of data
 */
static int subbuf_splice_actor(struct file *in,
                         loff_t *ppos,
                         struct pipe_inode_info *pipe,
                         size_t len,
                         unsigned int flags,
                         int *nonpad_ret)
{
      unsigned int pidx, poff, total_len, subbuf_pages, ret;
      struct rchan_buf *rbuf = in->private_data;
      unsigned int subbuf_size = rbuf->chan->subbuf_size;
      uint64_t pos = (uint64_t) *ppos;
      uint32_t alloc_size = (uint32_t) rbuf->chan->alloc_size;
      size_t read_start = (size_t) do_div(pos, alloc_size);
      size_t read_subbuf = read_start / subbuf_size;
      size_t padding = rbuf->padding[read_subbuf];
      size_t nonpad_end = read_subbuf * subbuf_size + subbuf_size - padding;
      struct page *pages[PIPE_BUFFERS];
      struct partial_page partial[PIPE_BUFFERS];
      struct splice_pipe_desc spd = {
            .pages = pages,
            .nr_pages = 0,
            .partial = partial,
            .flags = flags,
            .ops = &relay_pipe_buf_ops,
      };

      if (rbuf->subbufs_produced == rbuf->subbufs_consumed)
            return 0;

      /*
       * Adjust read len, if longer than what is available
       */
      if (len > (subbuf_size - read_start % subbuf_size))
            len = subbuf_size - read_start % subbuf_size;

      subbuf_pages = rbuf->chan->alloc_size >> PAGE_SHIFT;
      pidx = (read_start / PAGE_SIZE) % subbuf_pages;
      poff = read_start & ~PAGE_MASK;

      for (total_len = 0; spd.nr_pages < subbuf_pages; spd.nr_pages++) {
            unsigned int this_len, this_end, private;
            unsigned int cur_pos = read_start + total_len;

            if (!len)
                  break;

            this_len = min_t(unsigned long, len, PAGE_SIZE - poff);
            private = this_len;

            spd.pages[spd.nr_pages] = rbuf->page_array[pidx];
            spd.partial[spd.nr_pages].offset = poff;

            this_end = cur_pos + this_len;
            if (this_end >= nonpad_end) {
                  this_len = nonpad_end - cur_pos;
                  private = this_len + padding;
            }
            spd.partial[spd.nr_pages].len = this_len;
            spd.partial[spd.nr_pages].private = private;

            len -= this_len;
            total_len += this_len;
            poff = 0;
            pidx = (pidx + 1) % subbuf_pages;

            if (this_end >= nonpad_end) {
                  spd.nr_pages++;
                  break;
            }
      }

      if (!spd.nr_pages)
            return 0;

      ret = *nonpad_ret = splice_to_pipe(pipe, &spd);
      if (ret < 0 || ret < total_len)
            return ret;

        if (read_start + ret == nonpad_end)
                ret += padding;

        return ret;
}

static ssize_t relay_file_splice_read(struct file *in,
                              loff_t *ppos,
                              struct pipe_inode_info *pipe,
                              size_t len,
                              unsigned int flags)
{
      ssize_t spliced;
      int ret;
      int nonpad_ret = 0;

      ret = 0;
      spliced = 0;

      while (len) {
            ret = subbuf_splice_actor(in, ppos, pipe, len, flags, &nonpad_ret);
            if (ret < 0)
                  break;
            else if (!ret) {
                  if (spliced)
                        break;
                  if (flags & SPLICE_F_NONBLOCK) {
                        ret = -EAGAIN;
                        break;
                  }
            }

            *ppos += ret;
            if (ret > len)
                  len = 0;
            else
                  len -= ret;
            spliced += nonpad_ret;
            nonpad_ret = 0;
      }

      if (spliced)
            return spliced;

      return ret;
}

const struct file_operations relay_file_operations = {
      .open       = relay_file_open,
      .poll       = relay_file_poll,
      .mmap       = relay_file_mmap,
      .read       = relay_file_read,
      .llseek           = no_llseek,
      .release    = relay_file_release,
      .splice_read      = relay_file_splice_read,
};
EXPORT_SYMBOL_GPL(relay_file_operations);

static __init int relay_init(void)
{

      hotcpu_notifier(relay_hotcpu_callback, 0);
      return 0;
}

module_init(relay_init);

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