backing-dev.c

Go to the documentation of this file.

#include <linux/wait.h>
#include <linux/backing-dev.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/writeback.h>
#include <linux/device.h>
#include <trace/events/writeback.h>

static atomic_long_t bdi_seq = ATOMIC_LONG_INIT(0);

struct backing_dev_info default_backing_dev_info = {
    .name       = "default",
    .ra_pages   = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
    .state      = 0,
    .capabilities   = BDI_CAP_MAP_COPY,
};
EXPORT_SYMBOL_GPL(default_backing_dev_info);

struct backing_dev_info noop_backing_dev_info = {
    .name       = "noop",
    .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK,
};
EXPORT_SYMBOL_GPL(noop_backing_dev_info);

static struct class *bdi_class;

/*
 * bdi_lock protects updates to bdi_list and bdi_pending_list, as well as
 * reader side protection for bdi_pending_list. bdi_list has RCU reader side
 * locking.
 */
DEFINE_SPINLOCK(bdi_lock);
LIST_HEAD(bdi_list);
LIST_HEAD(bdi_pending_list);

void bdi_lock_two(struct bdi_writeback *wb1, struct bdi_writeback *wb2)
{
    if (wb1 < wb2) {
        spin_lock(&wb1->list_lock);
        spin_lock_nested(&wb2->list_lock, 1);
    } else {
        spin_lock(&wb2->list_lock);
        spin_lock_nested(&wb1->list_lock, 1);
    }
}

#ifdef CONFIG_DEBUG_FS
#include <linux/debugfs.h>
#include <linux/seq_file.h>

static struct dentry *bdi_debug_root;

static void bdi_debug_init(void)
{
    bdi_debug_root = debugfs_create_dir("bdi", NULL);
}

static int bdi_debug_stats_show(struct seq_file *m, void *v)
{
    struct backing_dev_info *bdi = m->private;
    struct bdi_writeback *wb = &bdi->wb;
    unsigned long background_thresh;
    unsigned long dirty_thresh;
    unsigned long bdi_thresh;
    unsigned long nr_dirty, nr_io, nr_more_io;
    struct inode *inode;

    nr_dirty = nr_io = nr_more_io = 0;
    spin_lock(&wb->list_lock);
    list_for_each_entry(inode, &wb->b_dirty, i_wb_list)
        nr_dirty++;
    list_for_each_entry(inode, &wb->b_io, i_wb_list)
        nr_io++;
    list_for_each_entry(inode, &wb->b_more_io, i_wb_list)
        nr_more_io++;
    spin_unlock(&wb->list_lock);

    global_dirty_limits(&background_thresh, &dirty_thresh);
    bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);

#define K(x) ((x) << (PAGE_SHIFT - 10))
    seq_printf(m,
           "BdiWriteback:       %10lu kB\n"
           "BdiReclaimable:     %10lu kB\n"
           "BdiDirtyThresh:     %10lu kB\n"
           "DirtyThresh:        %10lu kB\n"
           "BackgroundThresh:   %10lu kB\n"
           "BdiDirtied:         %10lu kB\n"
           "BdiWritten:         %10lu kB\n"
           "BdiWriteBandwidth:  %10lu kBps\n"
           "b_dirty:            %10lu\n"
           "b_io:               %10lu\n"
           "b_more_io:          %10lu\n"
           "bdi_list:           %10u\n"
           "state:              %10lx\n",
           (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
           (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
           K(bdi_thresh),
           K(dirty_thresh),
           K(background_thresh),
           (unsigned long) K(bdi_stat(bdi, BDI_DIRTIED)),
           (unsigned long) K(bdi_stat(bdi, BDI_WRITTEN)),
           (unsigned long) K(bdi->write_bandwidth),
           nr_dirty,
           nr_io,
           nr_more_io,
           !list_empty(&bdi->bdi_list), bdi->state);
#undef K

    return 0;
}

static int bdi_debug_stats_open(struct inode *inode, struct file *file)
{
    return single_open(file, bdi_debug_stats_show, inode->i_private);
}

static const struct file_operations bdi_debug_stats_fops = {
    .open       = bdi_debug_stats_open,
    .read       = seq_read,
    .llseek     = seq_lseek,
    .release    = single_release,
};

static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
{
    bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
    bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
                           bdi, &bdi_debug_stats_fops);
}

static void bdi_debug_unregister(struct backing_dev_info *bdi)
{
    debugfs_remove(bdi->debug_stats);
    debugfs_remove(bdi->debug_dir);
}
#else
static inline void bdi_debug_init(void)
{
}
static inline void bdi_debug_register(struct backing_dev_info *bdi,
                      const char *name)
{
}
static inline void bdi_debug_unregister(struct backing_dev_info *bdi)
{
}
#endif

static ssize_t read_ahead_kb_store(struct device *dev,
                  struct device_attribute *attr,
                  const char *buf, size_t count)
{
    struct backing_dev_info *bdi = dev_get_drvdata(dev);
    unsigned long read_ahead_kb;
    ssize_t ret;

    ret = kstrtoul(buf, 10, &read_ahead_kb);
    if (ret < 0)
        return ret;

    bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);

    return count;
}

#define K(pages) ((pages) << (PAGE_SHIFT - 10))

#define BDI_SHOW(name, expr)                        \
static ssize_t name##_show(struct device *dev,              \
               struct device_attribute *attr, char *page)   \
{                                   \
    struct backing_dev_info *bdi = dev_get_drvdata(dev);        \
                                    \
    return snprintf(page, PAGE_SIZE-1, "%lld\n", (long long)expr);  \
}

BDI_SHOW(read_ahead_kb, K(bdi->ra_pages))

static ssize_t min_ratio_store(struct device *dev,
        struct device_attribute *attr, const char *buf, size_t count)
{
    struct backing_dev_info *bdi = dev_get_drvdata(dev);
    unsigned int ratio;
    ssize_t ret;

    ret = kstrtouint(buf, 10, &ratio);
    if (ret < 0)
        return ret;

    ret = bdi_set_min_ratio(bdi, ratio);
    if (!ret)
        ret = count;

    return ret;
}
BDI_SHOW(min_ratio, bdi->min_ratio)

static ssize_t max_ratio_store(struct device *dev,
        struct device_attribute *attr, const char *buf, size_t count)
{
    struct backing_dev_info *bdi = dev_get_drvdata(dev);
    unsigned int ratio;
    ssize_t ret;

    ret = kstrtouint(buf, 10, &ratio);
    if (ret < 0)
        return ret;

    ret = bdi_set_max_ratio(bdi, ratio);
    if (!ret)
        ret = count;

    return ret;
}
BDI_SHOW(max_ratio, bdi->max_ratio)

#define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)

static struct device_attribute bdi_dev_attrs[] = {
    __ATTR_RW(read_ahead_kb),
    __ATTR_RW(min_ratio),
    __ATTR_RW(max_ratio),
    __ATTR_NULL,
};

static __init int bdi_class_init(void)
{
    bdi_class = class_create(THIS_MODULE, "bdi");
    if (IS_ERR(bdi_class))
        return PTR_ERR(bdi_class);

    bdi_class->dev_attrs = bdi_dev_attrs;
    bdi_debug_init();
    return 0;
}
postcore_initcall(bdi_class_init);

static int __init default_bdi_init(void)
{
    int err;

    err = bdi_init(&default_backing_dev_info);
    if (!err)
        bdi_register(&default_backing_dev_info, NULL, "default");
    err = bdi_init(&noop_backing_dev_info);

    return err;
}
subsys_initcall(default_bdi_init);

int bdi_has_dirty_io(struct backing_dev_info *bdi)
{
    return wb_has_dirty_io(&bdi->wb);
}

static void wakeup_timer_fn(unsigned long data)
{
    struct backing_dev_info *bdi = (struct backing_dev_info *)data;

    spin_lock_bh(&bdi->wb_lock);
    if (bdi->wb.task) {
        trace_writeback_wake_thread(bdi);
        wake_up_process(bdi->wb.task);
    } else if (bdi->dev) {
        /*
         * When bdi tasks are inactive for long time, they are killed.
         * In this case we have to wake-up the forker thread which
         * should create and run the bdi thread.
         */
        trace_writeback_wake_forker_thread(bdi);
        wake_up_process(default_backing_dev_info.wb.task);
    }
    spin_unlock_bh(&bdi->wb_lock);
}

/*
 * This function is used when the first inode for this bdi is marked dirty. It
 * wakes-up the corresponding bdi thread which should then take care of the
 * periodic background write-out of dirty inodes. Since the write-out would
 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
 * set up a timer which wakes the bdi thread up later.
 *
 * Note, we wouldn't bother setting up the timer, but this function is on the
 * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
 * by delaying the wake-up.
 */
void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
{
    unsigned long timeout;

    timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
    mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
}

/*
 * Calculate the longest interval (jiffies) bdi threads are allowed to be
 * inactive.
 */
static unsigned long bdi_longest_inactive(void)
{
    unsigned long interval;

    interval = msecs_to_jiffies(dirty_writeback_interval * 10);
    return max(5UL * 60 * HZ, interval);
}

/*
 * Clear pending bit and wakeup anybody waiting for flusher thread creation or
 * shutdown
 */
static void bdi_clear_pending(struct backing_dev_info *bdi)
{
    clear_bit(BDI_pending, &bdi->state);
    smp_mb__after_clear_bit();
    wake_up_bit(&bdi->state, BDI_pending);
}

static int bdi_forker_thread(void *ptr)
{
    struct bdi_writeback *me = ptr;

    current->flags |= PF_SWAPWRITE;
    set_freezable();

    /*
     * Our parent may run at a different priority, just set us to normal
     */
    set_user_nice(current, 0);

    for (;;) {
        struct task_struct *task = NULL;
        struct backing_dev_info *bdi;
        enum {
            NO_ACTION,   /* Nothing to do */
            FORK_THREAD, /* Fork bdi thread */
            KILL_THREAD, /* Kill inactive bdi thread */
        } action = NO_ACTION;

        /*
         * Temporary measure, we want to make sure we don't see
         * dirty data on the default backing_dev_info
         */
        if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {
            del_timer(&me->wakeup_timer);
            wb_do_writeback(me, 0);
        }

        spin_lock_bh(&bdi_lock);
        /*
         * In the following loop we are going to check whether we have
         * some work to do without any synchronization with tasks
         * waking us up to do work for them. Set the task state here
         * so that we don't miss wakeups after verifying conditions.
         */
        set_current_state(TASK_INTERRUPTIBLE);

        list_for_each_entry(bdi, &bdi_list, bdi_list) {
            bool have_dirty_io;

            if (!bdi_cap_writeback_dirty(bdi) ||
                 bdi_cap_flush_forker(bdi))
                continue;

            WARN(!test_bit(BDI_registered, &bdi->state),
                 "bdi %p/%s is not registered!\n", bdi, bdi->name);

            have_dirty_io = !list_empty(&bdi->work_list) ||
                    wb_has_dirty_io(&bdi->wb);

            /*
             * If the bdi has work to do, but the thread does not
             * exist - create it.
             */
            if (!bdi->wb.task && have_dirty_io) {
                /*
                 * Set the pending bit - if someone will try to
                 * unregister this bdi - it'll wait on this bit.
                 */
                set_bit(BDI_pending, &bdi->state);
                action = FORK_THREAD;
                break;
            }

            spin_lock(&bdi->wb_lock);

            /*
             * If there is no work to do and the bdi thread was
             * inactive long enough - kill it. The wb_lock is taken
             * to make sure no-one adds more work to this bdi and
             * wakes the bdi thread up.
             */
            if (bdi->wb.task && !have_dirty_io &&
                time_after(jiffies, bdi->wb.last_active +
                        bdi_longest_inactive())) {
                task = bdi->wb.task;
                bdi->wb.task = NULL;
                spin_unlock(&bdi->wb_lock);
                set_bit(BDI_pending, &bdi->state);
                action = KILL_THREAD;
                break;
            }
            spin_unlock(&bdi->wb_lock);
        }
        spin_unlock_bh(&bdi_lock);

        /* Keep working if default bdi still has things to do */
        if (!list_empty(&me->bdi->work_list))
            __set_current_state(TASK_RUNNING);

        switch (action) {
        case FORK_THREAD:
            __set_current_state(TASK_RUNNING);
            task = kthread_create(bdi_writeback_thread, &bdi->wb,
                          "flush-%s", dev_name(bdi->dev));
            if (IS_ERR(task)) {
                /*
                 * If thread creation fails, force writeout of
                 * the bdi from the thread. Hopefully 1024 is
                 * large enough for efficient IO.
                 */
                writeback_inodes_wb(&bdi->wb, 1024,
                            WB_REASON_FORKER_THREAD);
            } else {
                /*
                 * The spinlock makes sure we do not lose
                 * wake-ups when racing with 'bdi_queue_work()'.
                 * And as soon as the bdi thread is visible, we
                 * can start it.
                 */
                spin_lock_bh(&bdi->wb_lock);
                bdi->wb.task = task;
                spin_unlock_bh(&bdi->wb_lock);
                wake_up_process(task);
            }
            bdi_clear_pending(bdi);
            break;

        case KILL_THREAD:
            __set_current_state(TASK_RUNNING);
            kthread_stop(task);
            bdi_clear_pending(bdi);
            break;

        case NO_ACTION:
            if (!wb_has_dirty_io(me) || !dirty_writeback_interval)
                /*
                 * There are no dirty data. The only thing we
                 * should now care about is checking for
                 * inactive bdi threads and killing them. Thus,
                 * let's sleep for longer time, save energy and
                 * be friendly for battery-driven devices.
                 */
                schedule_timeout(bdi_longest_inactive());
            else
                schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
            try_to_freeze();
            break;
        }
    }

    return 0;
}

/*
 * Remove bdi from bdi_list, and ensure that it is no longer visible
 */
static void bdi_remove_from_list(struct backing_dev_info *bdi)
{
    spin_lock_bh(&bdi_lock);
    list_del_rcu(&bdi->bdi_list);
    spin_unlock_bh(&bdi_lock);

    synchronize_rcu_expedited();
}

int bdi_register(struct backing_dev_info *bdi, struct device *parent,
        const char *fmt, ...)
{
    va_list args;
    struct device *dev;

    if (bdi->dev)   /* The driver needs to use separate queues per device */
        return 0;

    va_start(args, fmt);
    dev = device_create_vargs(bdi_class, parent, MKDEV(0, 0), bdi, fmt, args);
    va_end(args);
    if (IS_ERR(dev))
        return PTR_ERR(dev);

    bdi->dev = dev;

    /*
     * Just start the forker thread for our default backing_dev_info,
     * and add other bdi's to the list. They will get a thread created
     * on-demand when they need it.
     */
    if (bdi_cap_flush_forker(bdi)) {
        struct bdi_writeback *wb = &bdi->wb;

        wb->task = kthread_run(bdi_forker_thread, wb, "bdi-%s",
                        dev_name(dev));
        if (IS_ERR(wb->task))
            return PTR_ERR(wb->task);
    }

    bdi_debug_register(bdi, dev_name(dev));
    set_bit(BDI_registered, &bdi->state);

    spin_lock_bh(&bdi_lock);
    list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
    spin_unlock_bh(&bdi_lock);

    trace_writeback_bdi_register(bdi);
    return 0;
}
EXPORT_SYMBOL(bdi_register);

int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev)
{
    return bdi_register(bdi, NULL, "%u:%u", MAJOR(dev), MINOR(dev));
}
EXPORT_SYMBOL(bdi_register_dev);

/*
 * Remove bdi from the global list and shutdown any threads we have running
 */
static void bdi_wb_shutdown(struct backing_dev_info *bdi)
{
    struct task_struct *task;

    if (!bdi_cap_writeback_dirty(bdi))
        return;

    /*
     * Make sure nobody finds us on the bdi_list anymore
     */
    bdi_remove_from_list(bdi);

    /*
     * If setup is pending, wait for that to complete first
     */
    wait_on_bit(&bdi->state, BDI_pending, bdi_sched_wait,
            TASK_UNINTERRUPTIBLE);

    /*
     * Finally, kill the kernel thread. We don't need to be RCU
     * safe anymore, since the bdi is gone from visibility.
     */
    spin_lock_bh(&bdi->wb_lock);
    task = bdi->wb.task;
    bdi->wb.task = NULL;
    spin_unlock_bh(&bdi->wb_lock);

    if (task)
        kthread_stop(task);
}

/*
 * This bdi is going away now, make sure that no super_blocks point to it
 */
static void bdi_prune_sb(struct backing_dev_info *bdi)
{
    struct super_block *sb;

    spin_lock(&sb_lock);
    list_for_each_entry(sb, &super_blocks, s_list) {
        if (sb->s_bdi == bdi)
            sb->s_bdi = &default_backing_dev_info;
    }
    spin_unlock(&sb_lock);
}

void bdi_unregister(struct backing_dev_info *bdi)
{
    struct device *dev = bdi->dev;

    if (dev) {
        bdi_set_min_ratio(bdi, 0);
        trace_writeback_bdi_unregister(bdi);
        bdi_prune_sb(bdi);
        del_timer_sync(&bdi->wb.wakeup_timer);

        if (!bdi_cap_flush_forker(bdi))
            bdi_wb_shutdown(bdi);
        bdi_debug_unregister(bdi);

        spin_lock_bh(&bdi->wb_lock);
        bdi->dev = NULL;
        spin_unlock_bh(&bdi->wb_lock);

        device_unregister(dev);
    }
}
EXPORT_SYMBOL(bdi_unregister);

static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
{
    memset(wb, 0, sizeof(*wb));

    wb->bdi = bdi;
    wb->last_old_flush = jiffies;
    INIT_LIST_HEAD(&wb->b_dirty);
    INIT_LIST_HEAD(&wb->b_io);
    INIT_LIST_HEAD(&wb->b_more_io);
    spin_lock_init(&wb->list_lock);
    setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);
}

/*
 * Initial write bandwidth: 100 MB/s
 */
#define INIT_BW     (100 << (20 - PAGE_SHIFT))

int bdi_init(struct backing_dev_info *bdi)
{
    int i, err;

    bdi->dev = NULL;

    bdi->min_ratio = 0;
    bdi->max_ratio = 100;
    bdi->max_prop_frac = FPROP_FRAC_BASE;
    spin_lock_init(&bdi->wb_lock);
    INIT_LIST_HEAD(&bdi->bdi_list);
    INIT_LIST_HEAD(&bdi->work_list);

    bdi_wb_init(&bdi->wb, bdi);

    for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
        err = percpu_counter_init(&bdi->bdi_stat[i], 0);
        if (err)
            goto err;
    }

    bdi->dirty_exceeded = 0;

    bdi->bw_time_stamp = jiffies;
    bdi->written_stamp = 0;

    bdi->balanced_dirty_ratelimit = INIT_BW;
    bdi->dirty_ratelimit = INIT_BW;
    bdi->write_bandwidth = INIT_BW;
    bdi->avg_write_bandwidth = INIT_BW;

    err = fprop_local_init_percpu(&bdi->completions);

    if (err) {
err:
        while (i--)
            percpu_counter_destroy(&bdi->bdi_stat[i]);
    }

    return err;
}
EXPORT_SYMBOL(bdi_init);

void bdi_destroy(struct backing_dev_info *bdi)
{
    int i;

    /*
     * Splice our entries to the default_backing_dev_info, if this
     * bdi disappears
     */
    if (bdi_has_dirty_io(bdi)) {
        struct bdi_writeback *dst = &default_backing_dev_info.wb;

        bdi_lock_two(&bdi->wb, dst);
        list_splice(&bdi->wb.b_dirty, &dst->b_dirty);
        list_splice(&bdi->wb.b_io, &dst->b_io);
        list_splice(&bdi->wb.b_more_io, &dst->b_more_io);
        spin_unlock(&bdi->wb.list_lock);
        spin_unlock(&dst->list_lock);
    }

    bdi_unregister(bdi);

    /*
     * If bdi_unregister() had already been called earlier, the
     * wakeup_timer could still be armed because bdi_prune_sb()
     * can race with the bdi_wakeup_thread_delayed() calls from
     * __mark_inode_dirty().
     */
    del_timer_sync(&bdi->wb.wakeup_timer);

    for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
        percpu_counter_destroy(&bdi->bdi_stat[i]);

    fprop_local_destroy_percpu(&bdi->completions);
}
EXPORT_SYMBOL(bdi_destroy);

/*
 * For use from filesystems to quickly init and register a bdi associated
 * with dirty writeback
 */
int bdi_setup_and_register(struct backing_dev_info *bdi, char *name,
               unsigned int cap)
{
    char tmp[32];
    int err;

    bdi->name = name;
    bdi->capabilities = cap;
    err = bdi_init(bdi);
    if (err)
        return err;

    sprintf(tmp, "%.28s%s", name, "-%d");
    err = bdi_register(bdi, NULL, tmp, atomic_long_inc_return(&bdi_seq));
    if (err) {
        bdi_destroy(bdi);
        return err;
    }

    return 0;
}
EXPORT_SYMBOL(bdi_setup_and_register);

static wait_queue_head_t congestion_wqh[2] = {
        __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[0]),
        __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
    };
static atomic_t nr_bdi_congested[2];

void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
{
    enum bdi_state bit;
    wait_queue_head_t *wqh = &congestion_wqh[sync];

    bit = sync ? BDI_sync_congested : BDI_async_congested;
    if (test_and_clear_bit(bit, &bdi->state))
        atomic_dec(&nr_bdi_congested[sync]);
    smp_mb__after_clear_bit();
    if (waitqueue_active(wqh))
        wake_up(wqh);
}
EXPORT_SYMBOL(clear_bdi_congested);

void set_bdi_congested(struct backing_dev_info *bdi, int sync)
{
    enum bdi_state bit;

    bit = sync ? BDI_sync_congested : BDI_async_congested;
    if (!test_and_set_bit(bit, &bdi->state))
        atomic_inc(&nr_bdi_congested[sync]);
}
EXPORT_SYMBOL(set_bdi_congested);

long congestion_wait(int sync, long timeout)
{
    long ret;
    unsigned long start = jiffies;
    DEFINE_WAIT(wait);
    wait_queue_head_t *wqh = &congestion_wqh[sync];

    prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
    ret = io_schedule_timeout(timeout);
    finish_wait(wqh, &wait);

    trace_writeback_congestion_wait(jiffies_to_usecs(timeout),
                    jiffies_to_usecs(jiffies - start));

    return ret;
}
EXPORT_SYMBOL(congestion_wait);

long wait_iff_congested(struct zone *zone, int sync, long timeout)
{
    long ret;
    unsigned long start = jiffies;
    DEFINE_WAIT(wait);
    wait_queue_head_t *wqh = &congestion_wqh[sync];

    /*
     * If there is no congestion, or heavy congestion is not being
     * encountered in the current zone, yield if necessary instead
     * of sleeping on the congestion queue
     */
    if (atomic_read(&nr_bdi_congested[sync]) == 0 ||
            !zone_is_reclaim_congested(zone)) {
        cond_resched();

        /* In case we scheduled, work out time remaining */
        ret = timeout - (jiffies - start);
        if (ret < 0)
            ret = 0;

        goto out;
    }

    /* Sleep until uncongested or a write happens */
    prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
    ret = io_schedule_timeout(timeout);
    finish_wait(wqh, &wait);

out:
    trace_writeback_wait_iff_congested(jiffies_to_usecs(timeout),
                    jiffies_to_usecs(jiffies - start));

    return ret;
}
EXPORT_SYMBOL(wait_iff_congested);

int pdflush_proc_obsolete(struct ctl_table *table, int write,
            void __user *buffer, size_t *lenp, loff_t *ppos)
{
    char kbuf[] = "0\n";

    if (*ppos) {
        *lenp = 0;
        return 0;
    }

    if (copy_to_user(buffer, kbuf, sizeof(kbuf)))
        return -EFAULT;
    printk_once(KERN_WARNING "%s exported in /proc is scheduled for removal\n",
            table->procname);

    *lenp = 2;
    *ppos += *lenp;
    return 2;
}