relay.c

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/*
 * 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 ([email protected]), IBM Corp
 * Copyright (C) 1999-2005 - Karim Yaghmour ([email protected])
 *
 * Moved to kernel/relay.c by Paul Mundt, 2006.
 * November 2006 - CPU hotplug support by Mathieu Desnoyers
 *  ([email protected])
 *
 * This file is released under the GPL.
 */
#include <linux/errno.h>
#include <linux/stddef.h>
#include <linux/slab.h>
#include <linux/export.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);
}

/*
 * fault() vm_op implementation for relay file mapping.
 */
static int relay_buf_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
    struct page *page;
    struct rchan_buf *buf = vma->vm_private_data;
    pgoff_t pgoff = vmf->pgoff;

    if (!buf)
        return VM_FAULT_OOM;

    page = vmalloc_to_page(buf->start + (pgoff << PAGE_SHIFT));
    if (!page)
        return VM_FAULT_SIGBUS;
    get_page(page);
    vmf->page = page;

    return 0;
}

/*
 * vm_ops for relay file mappings.
 */
static const struct vm_operations_struct relay_file_mmap_ops = {
    .fault = relay_buf_fault,
    .close = relay_file_mmap_close,
};

/*
 * allocate an array of pointers of struct page
 */
static struct page **relay_alloc_page_array(unsigned int n_pages)
{
    const size_t pa_size = n_pages * sizeof(struct page *);
    if (pa_size > PAGE_SIZE)
        return vzalloc(pa_size);
    return kzalloc(pa_size, GFP_KERNEL);
}

/*
 * free an array of pointers of struct page
 */
static void relay_free_page_array(struct page **array)
{
    if (is_vmalloc_addr(array))
        vfree(array);
    else
        kfree(array);
}

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;
}

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 = relay_alloc_page_array(n_pages);
    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]);
    relay_free_page_array(buf->page_array);
    return NULL;
}

static struct rchan_buf *relay_create_buf(struct rchan *chan)
{
    struct rchan_buf *buf;

    if (chan->n_subbufs > UINT_MAX / sizeof(size_t *))
        return NULL;

    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;
}

static void relay_destroy_channel(struct kref *kref)
{
    struct rchan *chan = container_of(kref, struct rchan, kref);
    kfree(chan);
}

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]);
        relay_free_page_array(buf->page_array);
    }
    chan->buf[buf->cpu] = NULL;
    kfree(buf->padding);
    kfree(buf);
    kref_put(&chan->kref, relay_destroy_channel);
}

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);
}

static int relay_buf_empty(struct rchan_buf *buf)
{
    return (buf->subbufs_produced - buf->subbufs_consumed) ? 0 : 1;
}

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,
                               umode_t 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,
};

static void wakeup_readers(unsigned long data)
{
    struct rchan_buf *buf = (struct rchan_buf *)data;
    wake_up_interruptible(&buf->read_wait);
}

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);
}

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

static inline void relay_set_buf_dentry(struct rchan_buf *buf,
                    struct dentry *dentry)
{
    buf->dentry = dentry;
    buf->dentry->d_inode->i_size = buf->early_bytes;
}

static struct dentry *relay_create_buf_file(struct rchan *chan,
                        struct rchan_buf *buf,
                        unsigned int cpu)
{
    struct dentry *dentry;
    char *tmpname;

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

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

    kfree(tmpname);

    return dentry;
}

/*
 *  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;

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

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

    if (chan->has_base_filename) {
        dentry = relay_create_buf_file(chan, buf, cpu);
        if (!dentry)
            goto free_buf;
        relay_set_buf_dentry(buf, dentry);
    }

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

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

    return buf;

free_buf:
    relay_destroy_buf(buf);
    return NULL;
}

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;
}

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 notifier_from_errno(-ENOMEM);
            }
        }
        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;
}

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 (!(subbuf_size && n_subbufs))
        return NULL;
    if (subbuf_size > UINT_MAX / 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;
    if (base_filename) {
        chan->has_base_filename = 1;
        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_possible_cpu(i) {
        if (chan->buf[i])
            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);

struct rchan_percpu_buf_dispatcher {
    struct rchan_buf *buf;
    struct dentry *dentry;
};

/* Called in atomic context. */
static void __relay_set_buf_dentry(void *info)
{
    struct rchan_percpu_buf_dispatcher *p = info;

    relay_set_buf_dentry(p->buf, p->dentry);
}

int relay_late_setup_files(struct rchan *chan,
               const char *base_filename,
               struct dentry *parent)
{
    int err = 0;
    unsigned int i, curr_cpu;
    unsigned long flags;
    struct dentry *dentry;
    struct rchan_percpu_buf_dispatcher disp;

    if (!chan || !base_filename)
        return -EINVAL;

    strlcpy(chan->base_filename, base_filename, NAME_MAX);

    mutex_lock(&relay_channels_mutex);
    /* Is chan already set up? */
    if (unlikely(chan->has_base_filename)) {
        mutex_unlock(&relay_channels_mutex);
        return -EEXIST;
    }
    chan->has_base_filename = 1;
    chan->parent = parent;
    curr_cpu = get_cpu();
    /*
     * The CPU hotplug notifier ran before us and created buffers with
     * no files associated. So it's safe to call relay_setup_buf_file()
     * on all currently online CPUs.
     */
    for_each_online_cpu(i) {
        if (unlikely(!chan->buf[i])) {
            WARN_ONCE(1, KERN_ERR "CPU has no buffer!\n");
            err = -EINVAL;
            break;
        }

        dentry = relay_create_buf_file(chan, chan->buf[i], i);
        if (unlikely(!dentry)) {
            err = -EINVAL;
            break;
        }

        if (curr_cpu == i) {
            local_irq_save(flags);
            relay_set_buf_dentry(chan->buf[i], dentry);
            local_irq_restore(flags);
        } else {
            disp.buf = chan->buf[i];
            disp.dentry = dentry;
            smp_mb();
            /* relay_channels_mutex must be held, so wait. */
            err = smp_call_function_single(i,
                               __relay_set_buf_dentry,
                               &disp, 1);
        }
        if (unlikely(err))
            break;
    }
    put_cpu();
    mutex_unlock(&relay_channels_mutex);

    return err;
}

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++;
        if (buf->dentry)
            buf->dentry->d_inode->i_size +=
                buf->chan->subbuf_size -
                buf->padding[old_subbuf];
        else
            buf->early_bytes += 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);

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] ||
                    subbufs_consumed > chan->n_subbufs)
        return;

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

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);

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);

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 nonseekable_open(inode, filp);
}

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);
}

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;
}

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->subbufs_produced == buf->subbufs_consumed &&
        buf->offset == buf->bytes_consumed)
        return;

    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);

    consumed = buf->subbufs_consumed;

    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) {
        if (buf->offset == subbuf_size &&
            buf->subbufs_produced > buf->subbufs_consumed)
            return 1;
        return 0;
    }

    return 1;
}

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;
}

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;
}

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 const 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,
};

static void relay_page_release(struct splice_pipe_desc *spd, unsigned int i)
{
}

/*
 *  subbuf_splice_actor - splice up to one subbuf's worth of data
 */
static ssize_t 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, nr_pages;
    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_DEF_BUFFERS];
    struct partial_page partial[PIPE_DEF_BUFFERS];
    struct splice_pipe_desc spd = {
        .pages = pages,
        .nr_pages = 0,
        .nr_pages_max = PIPE_DEF_BUFFERS,
        .partial = partial,
        .flags = flags,
        .ops = &relay_pipe_buf_ops,
        .spd_release = relay_page_release,
    };
    ssize_t ret;

    if (rbuf->subbufs_produced == rbuf->subbufs_consumed)
        return 0;
    if (splice_grow_spd(pipe, &spd))
        return -ENOMEM;

    /*
     * 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;
    nr_pages = min_t(unsigned int, subbuf_pages, pipe->buffers);

    for (total_len = 0; spd.nr_pages < nr_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;
        }
    }

    ret = 0;
    if (!spd.nr_pages)
        goto out;

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

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

out:
    splice_shrink_spd(&spd);
    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 && !spliced) {
        ret = subbuf_splice_actor(in, ppos, pipe, len, flags, &nonpad_ret);
        if (ret < 0)
            break;
        else if (!ret) {
            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;
}

early_initcall(relay_init);