[NET_SCHED]: Fix endless loops (part 4): HTB

[hh.org.git] / kernel / relay.c

/*
 * Public API and common code for kernel->userspace relay file support.
 *
 * See Documentation/filesystems/relayfs.txt for an overview of relayfs.
 *
 * 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.
 *
 * 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>

/*
 * 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.
 */
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_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;
        }
        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.
 */
struct rchan_buf *relay_create_buf(struct rchan *chan)
{
        struct rchan_buf *buf = kcalloc(1, 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().
 */
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
 */
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);
        }
        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().
 */
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.
 */
int relay_buf_empty(struct rchan_buf *buf)
{
        return (buf->subbufs_produced - buf->subbufs_consumed) ? 0 : 1;
}
EXPORT_SYMBOL_GPL(relay_buf_empty);

/**
 *      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
 *      @private: the channel buffer
 *
 *      This is the work function used to defer reader waking.  The
 *      reason waking is deferred is that calling directly from write
 *      causes problems if you're writing from say the scheduler.
 */
static void wakeup_readers(void *private)
{
        struct rchan_buf *buf = private;
        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 inline 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);
                INIT_WORK(&buf->wake_readers, NULL, NULL);
        } else {
                cancel_delayed_work(&buf->wake_readers);
                flush_scheduled_work();
        }

        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;
        struct rchan_buf *prev = NULL;

        if (!chan)
                return;

        for (i = 0; i < NR_CPUS; i++) {
                if (!chan->buf[i] || chan->buf[i] == prev)
                        break;
                __relay_reset(chan->buf[i], 0);
                prev = chan->buf[i];
        }
}
EXPORT_SYMBOL_GPL(relay_reset);

/*
 *      relay_open_buf - create a new relay channel buffer
 *
 *      Internal - used by relay_open().
 */
static struct rchan_buf *relay_open_buf(struct rchan *chan,
                                        const char *filename,
                                        struct dentry *parent,
                                        int *is_global)
{
        struct rchan_buf *buf;
        struct dentry *dentry;

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

        buf = relay_create_buf(chan);
        if (!buf)
                return NULL;

        /* Create file in fs */
        dentry = chan->cb->create_buf_file(filename, parent, S_IRUSR,
                                           buf, is_global);
        if (!dentry) {
                relay_destroy_buf(buf);
                return NULL;
        }

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

        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 inline void relay_close_buf(struct rchan_buf *buf)
{
        buf->finalized = 1;
        cancel_delayed_work(&buf->wake_readers);
        flush_scheduled_work();
        kref_put(&buf->kref, relay_remove_buf);
}

static inline 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_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
 *
 *      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)
{
        unsigned int i;
        struct rchan *chan;
        char *tmpname;
        int is_global = 0;

        if (!base_filename)
                return NULL;

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

        chan = kcalloc(1, 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);
        setup_callbacks(chan, cb);
        kref_init(&chan->kref);

        tmpname = kmalloc(NAME_MAX + 1, GFP_KERNEL);
        if (!tmpname)
                goto free_chan;

        for_each_online_cpu(i) {
                sprintf(tmpname, "%s%d", base_filename, i);
                chan->buf[i] = relay_open_buf(chan, tmpname, parent,
                                              &is_global);
                if (!chan->buf[i])
                        goto free_bufs;

                chan->buf[i]->cpu = i;
        }

        kfree(tmpname);
        return chan;

free_bufs:
        for (i = 0; i < NR_CPUS; i++) {
                if (!chan->buf[i])
                        break;
                relay_close_buf(chan->buf[i]);
                if (is_global)
                        break;
        }
        kfree(tmpname);

free_chan:
        kref_put(&chan->kref, relay_destroy_channel);
        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)) {
                        PREPARE_WORK(&buf->wake_readers, wakeup_readers, buf);
                        schedule_delayed_work(&buf->wake_readers, 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;
        struct rchan_buf *prev = NULL;

        if (!chan)
                return;

        for (i = 0; i < NR_CPUS; i++) {
                if (!chan->buf[i] || chan->buf[i] == prev)
                        break;
                relay_close_buf(chan->buf[i]);
                prev = 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);

        kref_put(&chan->kref, relay_destroy_channel);
}
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;
        struct rchan_buf *prev = NULL;

        if (!chan)
                return;

        for (i = 0; i < NR_CPUS; i++) {
                if (!chan->buf[i] || chan->buf[i] == prev)
                        break;
                relay_switch_subbuf(chan->buf[i], 0);
                prev = chan->buf[i];
        }
}
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;
        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) * n_subbufs;
                buf->subbufs_consumed = consumed;
        }
        
        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;

        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;
}

/*
 *      subbuf_send_actor - send up to one subbuf's worth of data
 */
static int subbuf_send_actor(size_t read_start,
                             struct rchan_buf *buf,
                             size_t avail,
                             read_descriptor_t *desc,
                             read_actor_t actor)
{
        unsigned long pidx, poff;
        unsigned int subbuf_pages;
        int ret = 0;

        subbuf_pages = buf->chan->alloc_size >> PAGE_SHIFT;
        pidx = (read_start / PAGE_SIZE) % subbuf_pages;
        poff = read_start & ~PAGE_MASK;
        while (avail) {
                struct page *p = buf->page_array[pidx];
                unsigned int len;

                len = PAGE_SIZE - poff;
                if (len > avail)
                        len = avail;

                len = actor(desc, p, poff, len);
                if (desc->error)
                        break;

                avail -= len;
                ret += len;
                poff = 0;
                pidx = (pidx + 1) % subbuf_pages;
        }

        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 inline 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_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_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 ssize_t relay_file_sendfile(struct file *filp,
                                   loff_t *ppos,
                                   size_t count,
                                   read_actor_t actor,
                                   void *target)
{
        read_descriptor_t desc;
        desc.written = 0;
        desc.count = count;
        desc.arg.data = target;
        desc.error = 0;
        return relay_file_read_subbufs(filp, ppos, subbuf_send_actor,
                                       actor, &desc);
}

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,
        .sendfile       = relay_file_sendfile,
};
EXPORT_SYMBOL_GPL(relay_file_operations);