page-writeback.c

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/*
 * mm/page-writeback.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <[email protected]>
 *
 * Contains functions related to writing back dirty pages at the
 * address_space level.
 *
 * 10Apr2002    Andrew Morton
 *      Initial version
 */

#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/swap.h>
#include <linux/slab.h>
#include <linux/pagemap.h>
#include <linux/writeback.h>
#include <linux/init.h>
#include <linux/backing-dev.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/blkdev.h>
#include <linux/mpage.h>
#include <linux/rmap.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/smp.h>
#include <linux/sysctl.h>
#include <linux/cpu.h>
#include <linux/syscalls.h>
#include <linux/buffer_head.h> /* __set_page_dirty_buffers */
#include <linux/pagevec.h>
#include <linux/timer.h>
#include <trace/events/writeback.h>

/*
 * Sleep at most 200ms at a time in balance_dirty_pages().
 */
#define MAX_PAUSE       max(HZ/5, 1)

/*
 * Try to keep balance_dirty_pages() call intervals higher than this many pages
 * by raising pause time to max_pause when falls below it.
 */
#define DIRTY_POLL_THRESH   (128 >> (PAGE_SHIFT - 10))

/*
 * Estimate write bandwidth at 200ms intervals.
 */
#define BANDWIDTH_INTERVAL  max(HZ/5, 1)

#define RATELIMIT_CALC_SHIFT    10

/*
 * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
 * will look to see if it needs to force writeback or throttling.
 */
static long ratelimit_pages = 32;

/* The following parameters are exported via /proc/sys/vm */

/*
 * Start background writeback (via writeback threads) at this percentage
 */
int dirty_background_ratio = 10;

/*
 * dirty_background_bytes starts at 0 (disabled) so that it is a function of
 * dirty_background_ratio * the amount of dirtyable memory
 */
unsigned long dirty_background_bytes;

/*
 * free highmem will not be subtracted from the total free memory
 * for calculating free ratios if vm_highmem_is_dirtyable is true
 */
int vm_highmem_is_dirtyable;

/*
 * The generator of dirty data starts writeback at this percentage
 */
int vm_dirty_ratio = 20;

/*
 * vm_dirty_bytes starts at 0 (disabled) so that it is a function of
 * vm_dirty_ratio * the amount of dirtyable memory
 */
unsigned long vm_dirty_bytes;

/*
 * The interval between `kupdate'-style writebacks
 */
unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */

EXPORT_SYMBOL_GPL(dirty_writeback_interval);

/*
 * The longest time for which data is allowed to remain dirty
 */
unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */

/*
 * Flag that makes the machine dump writes/reads and block dirtyings.
 */
int block_dump;

/*
 * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
 * a full sync is triggered after this time elapses without any disk activity.
 */
int laptop_mode;

EXPORT_SYMBOL(laptop_mode);

/* End of sysctl-exported parameters */

unsigned long global_dirty_limit;

/*
 * Scale the writeback cache size proportional to the relative writeout speeds.
 *
 * We do this by keeping a floating proportion between BDIs, based on page
 * writeback completions [end_page_writeback()]. Those devices that write out
 * pages fastest will get the larger share, while the slower will get a smaller
 * share.
 *
 * We use page writeout completions because we are interested in getting rid of
 * dirty pages. Having them written out is the primary goal.
 *
 * We introduce a concept of time, a period over which we measure these events,
 * because demand can/will vary over time. The length of this period itself is
 * measured in page writeback completions.
 *
 */
static struct fprop_global writeout_completions;

static void writeout_period(unsigned long t);
/* Timer for aging of writeout_completions */
static struct timer_list writeout_period_timer =
        TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0);
static unsigned long writeout_period_time = 0;

/*
 * Length of period for aging writeout fractions of bdis. This is an
 * arbitrarily chosen number. The longer the period, the slower fractions will
 * reflect changes in current writeout rate.
 */
#define VM_COMPLETIONS_PERIOD_LEN (3*HZ)

/*
 * Work out the current dirty-memory clamping and background writeout
 * thresholds.
 *
 * The main aim here is to lower them aggressively if there is a lot of mapped
 * memory around.  To avoid stressing page reclaim with lots of unreclaimable
 * pages.  It is better to clamp down on writers than to start swapping, and
 * performing lots of scanning.
 *
 * We only allow 1/2 of the currently-unmapped memory to be dirtied.
 *
 * We don't permit the clamping level to fall below 5% - that is getting rather
 * excessive.
 *
 * We make sure that the background writeout level is below the adjusted
 * clamping level.
 */

/*
 * In a memory zone, there is a certain amount of pages we consider
 * available for the page cache, which is essentially the number of
 * free and reclaimable pages, minus some zone reserves to protect
 * lowmem and the ability to uphold the zone's watermarks without
 * requiring writeback.
 *
 * This number of dirtyable pages is the base value of which the
 * user-configurable dirty ratio is the effictive number of pages that
 * are allowed to be actually dirtied.  Per individual zone, or
 * globally by using the sum of dirtyable pages over all zones.
 *
 * Because the user is allowed to specify the dirty limit globally as
 * absolute number of bytes, calculating the per-zone dirty limit can
 * require translating the configured limit into a percentage of
 * global dirtyable memory first.
 */

static unsigned long highmem_dirtyable_memory(unsigned long total)
{
#ifdef CONFIG_HIGHMEM
    int node;
    unsigned long x = 0;

    for_each_node_state(node, N_HIGH_MEMORY) {
        struct zone *z =
            &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];

        x += zone_page_state(z, NR_FREE_PAGES) +
             zone_reclaimable_pages(z) - z->dirty_balance_reserve;
    }
    /*
     * Make sure that the number of highmem pages is never larger
     * than the number of the total dirtyable memory. This can only
     * occur in very strange VM situations but we want to make sure
     * that this does not occur.
     */
    return min(x, total);
#else
    return 0;
#endif
}

static unsigned long global_dirtyable_memory(void)
{
    unsigned long x;

    x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -
        dirty_balance_reserve;

    if (!vm_highmem_is_dirtyable)
        x -= highmem_dirtyable_memory(x);

    return x + 1;   /* Ensure that we never return 0 */
}

/*
 * global_dirty_limits - background-writeback and dirty-throttling thresholds
 *
 * Calculate the dirty thresholds based on sysctl parameters
 * - vm.dirty_background_ratio  or  vm.dirty_background_bytes
 * - vm.dirty_ratio             or  vm.dirty_bytes
 * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
 * real-time tasks.
 */
void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
{
    unsigned long background;
    unsigned long dirty;
    unsigned long uninitialized_var(available_memory);
    struct task_struct *tsk;

    if (!vm_dirty_bytes || !dirty_background_bytes)
        available_memory = global_dirtyable_memory();

    if (vm_dirty_bytes)
        dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
    else
        dirty = (vm_dirty_ratio * available_memory) / 100;

    if (dirty_background_bytes)
        background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
    else
        background = (dirty_background_ratio * available_memory) / 100;

    if (background >= dirty)
        background = dirty / 2;
    tsk = current;
    if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
        background += background / 4;
        dirty += dirty / 4;
    }
    *pbackground = background;
    *pdirty = dirty;
    trace_global_dirty_state(background, dirty);
}

static unsigned long zone_dirtyable_memory(struct zone *zone)
{
    /*
     * The effective global number of dirtyable pages may exclude
     * highmem as a big-picture measure to keep the ratio between
     * dirty memory and lowmem reasonable.
     *
     * But this function is purely about the individual zone and a
     * highmem zone can hold its share of dirty pages, so we don't
     * care about vm_highmem_is_dirtyable here.
     */
    return zone_page_state(zone, NR_FREE_PAGES) +
           zone_reclaimable_pages(zone) -
           zone->dirty_balance_reserve;
}

static unsigned long zone_dirty_limit(struct zone *zone)
{
    unsigned long zone_memory = zone_dirtyable_memory(zone);
    struct task_struct *tsk = current;
    unsigned long dirty;

    if (vm_dirty_bytes)
        dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
            zone_memory / global_dirtyable_memory();
    else
        dirty = vm_dirty_ratio * zone_memory / 100;

    if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
        dirty += dirty / 4;

    return dirty;
}

bool zone_dirty_ok(struct zone *zone)
{
    unsigned long limit = zone_dirty_limit(zone);

    return zone_page_state(zone, NR_FILE_DIRTY) +
           zone_page_state(zone, NR_UNSTABLE_NFS) +
           zone_page_state(zone, NR_WRITEBACK) <= limit;
}

int dirty_background_ratio_handler(struct ctl_table *table, int write,
        void __user *buffer, size_t *lenp,
        loff_t *ppos)
{
    int ret;

    ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
    if (ret == 0 && write)
        dirty_background_bytes = 0;
    return ret;
}

int dirty_background_bytes_handler(struct ctl_table *table, int write,
        void __user *buffer, size_t *lenp,
        loff_t *ppos)
{
    int ret;

    ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
    if (ret == 0 && write)
        dirty_background_ratio = 0;
    return ret;
}

int dirty_ratio_handler(struct ctl_table *table, int write,
        void __user *buffer, size_t *lenp,
        loff_t *ppos)
{
    int old_ratio = vm_dirty_ratio;
    int ret;

    ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
    if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
        writeback_set_ratelimit();
        vm_dirty_bytes = 0;
    }
    return ret;
}

int dirty_bytes_handler(struct ctl_table *table, int write,
        void __user *buffer, size_t *lenp,
        loff_t *ppos)
{
    unsigned long old_bytes = vm_dirty_bytes;
    int ret;

    ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
    if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
        writeback_set_ratelimit();
        vm_dirty_ratio = 0;
    }
    return ret;
}

static unsigned long wp_next_time(unsigned long cur_time)
{
    cur_time += VM_COMPLETIONS_PERIOD_LEN;
    /* 0 has a special meaning... */
    if (!cur_time)
        return 1;
    return cur_time;
}

/*
 * Increment the BDI's writeout completion count and the global writeout
 * completion count. Called from test_clear_page_writeback().
 */
static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
{
    __inc_bdi_stat(bdi, BDI_WRITTEN);
    __fprop_inc_percpu_max(&writeout_completions, &bdi->completions,
                   bdi->max_prop_frac);
    /* First event after period switching was turned off? */
    if (!unlikely(writeout_period_time)) {
        /*
         * We can race with other __bdi_writeout_inc calls here but
         * it does not cause any harm since the resulting time when
         * timer will fire and what is in writeout_period_time will be
         * roughly the same.
         */
        writeout_period_time = wp_next_time(jiffies);
        mod_timer(&writeout_period_timer, writeout_period_time);
    }
}

void bdi_writeout_inc(struct backing_dev_info *bdi)
{
    unsigned long flags;

    local_irq_save(flags);
    __bdi_writeout_inc(bdi);
    local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(bdi_writeout_inc);

/*
 * Obtain an accurate fraction of the BDI's portion.
 */
static void bdi_writeout_fraction(struct backing_dev_info *bdi,
        long *numerator, long *denominator)
{
    fprop_fraction_percpu(&writeout_completions, &bdi->completions,
                numerator, denominator);
}

/*
 * On idle system, we can be called long after we scheduled because we use
 * deferred timers so count with missed periods.
 */
static void writeout_period(unsigned long t)
{
    int miss_periods = (jiffies - writeout_period_time) /
                         VM_COMPLETIONS_PERIOD_LEN;

    if (fprop_new_period(&writeout_completions, miss_periods + 1)) {
        writeout_period_time = wp_next_time(writeout_period_time +
                miss_periods * VM_COMPLETIONS_PERIOD_LEN);
        mod_timer(&writeout_period_timer, writeout_period_time);
    } else {
        /*
         * Aging has zeroed all fractions. Stop wasting CPU on period
         * updates.
         */
        writeout_period_time = 0;
    }
}

/*
 * bdi_min_ratio keeps the sum of the minimum dirty shares of all
 * registered backing devices, which, for obvious reasons, can not
 * exceed 100%.
 */
static unsigned int bdi_min_ratio;

int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
{
    int ret = 0;

    spin_lock_bh(&bdi_lock);
    if (min_ratio > bdi->max_ratio) {
        ret = -EINVAL;
    } else {
        min_ratio -= bdi->min_ratio;
        if (bdi_min_ratio + min_ratio < 100) {
            bdi_min_ratio += min_ratio;
            bdi->min_ratio += min_ratio;
        } else {
            ret = -EINVAL;
        }
    }
    spin_unlock_bh(&bdi_lock);

    return ret;
}

int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
{
    int ret = 0;

    if (max_ratio > 100)
        return -EINVAL;

    spin_lock_bh(&bdi_lock);
    if (bdi->min_ratio > max_ratio) {
        ret = -EINVAL;
    } else {
        bdi->max_ratio = max_ratio;
        bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
    }
    spin_unlock_bh(&bdi_lock);

    return ret;
}
EXPORT_SYMBOL(bdi_set_max_ratio);

static unsigned long dirty_freerun_ceiling(unsigned long thresh,
                       unsigned long bg_thresh)
{
    return (thresh + bg_thresh) / 2;
}

static unsigned long hard_dirty_limit(unsigned long thresh)
{
    return max(thresh, global_dirty_limit);
}

unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
{
    u64 bdi_dirty;
    long numerator, denominator;

    /*
     * Calculate this BDI's share of the dirty ratio.
     */
    bdi_writeout_fraction(bdi, &numerator, &denominator);

    bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
    bdi_dirty *= numerator;
    do_div(bdi_dirty, denominator);

    bdi_dirty += (dirty * bdi->min_ratio) / 100;
    if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
        bdi_dirty = dirty * bdi->max_ratio / 100;

    return bdi_dirty;
}

/*
 * Dirty position control.
 *
 * (o) global/bdi setpoints
 *
 * We want the dirty pages be balanced around the global/bdi setpoints.
 * When the number of dirty pages is higher/lower than the setpoint, the
 * dirty position control ratio (and hence task dirty ratelimit) will be
 * decreased/increased to bring the dirty pages back to the setpoint.
 *
 *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
 *
 *     if (dirty < setpoint) scale up   pos_ratio
 *     if (dirty > setpoint) scale down pos_ratio
 *
 *     if (bdi_dirty < bdi_setpoint) scale up   pos_ratio
 *     if (bdi_dirty > bdi_setpoint) scale down pos_ratio
 *
 *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
 *
 * (o) global control line
 *
 *     ^ pos_ratio
 *     |
 *     |            |<===== global dirty control scope ======>|
 * 2.0 .............*
 *     |            .*
 *     |            . *
 *     |            .   *
 *     |            .     *
 *     |            .        *
 *     |            .            *
 * 1.0 ................................*
 *     |            .                  .     *
 *     |            .                  .          *
 *     |            .                  .              *
 *     |            .                  .                 *
 *     |            .                  .                    *
 *   0 +------------.------------------.----------------------*------------->
 *           freerun^          setpoint^                 limit^   dirty pages
 *
 * (o) bdi control line
 *
 *     ^ pos_ratio
 *     |
 *     |            *
 *     |              *
 *     |                *
 *     |                  *
 *     |                    * |<=========== span ============>|
 * 1.0 .......................*
 *     |                      . *
 *     |                      .   *
 *     |                      .     *
 *     |                      .       *
 *     |                      .         *
 *     |                      .           *
 *     |                      .             *
 *     |                      .               *
 *     |                      .                 *
 *     |                      .                   *
 *     |                      .                     *
 * 1/4 ...............................................* * * * * * * * * * * *
 *     |                      .                         .
 *     |                      .                           .
 *     |                      .                             .
 *   0 +----------------------.-------------------------------.------------->
 *                bdi_setpoint^                    x_intercept^
 *
 * The bdi control line won't drop below pos_ratio=1/4, so that bdi_dirty can
 * be smoothly throttled down to normal if it starts high in situations like
 * - start writing to a slow SD card and a fast disk at the same time. The SD
 *   card's bdi_dirty may rush to many times higher than bdi_setpoint.
 * - the bdi dirty thresh drops quickly due to change of JBOD workload
 */
static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
                    unsigned long thresh,
                    unsigned long bg_thresh,
                    unsigned long dirty,
                    unsigned long bdi_thresh,
                    unsigned long bdi_dirty)
{
    unsigned long write_bw = bdi->avg_write_bandwidth;
    unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
    unsigned long limit = hard_dirty_limit(thresh);
    unsigned long x_intercept;
    unsigned long setpoint;     /* dirty pages' target balance point */
    unsigned long bdi_setpoint;
    unsigned long span;
    long long pos_ratio;        /* for scaling up/down the rate limit */
    long x;

    if (unlikely(dirty >= limit))
        return 0;

    /*
     * global setpoint
     *
     *                           setpoint - dirty 3
     *        f(dirty) := 1.0 + (----------------)
     *                           limit - setpoint
     *
     * it's a 3rd order polynomial that subjects to
     *
     * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
     * (2) f(setpoint) = 1.0 => the balance point
     * (3) f(limit)    = 0   => the hard limit
     * (4) df/dx      <= 0   => negative feedback control
     * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
     *     => fast response on large errors; small oscillation near setpoint
     */
    setpoint = (freerun + limit) / 2;
    x = div_s64((setpoint - dirty) << RATELIMIT_CALC_SHIFT,
            limit - setpoint + 1);
    pos_ratio = x;
    pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
    pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
    pos_ratio += 1 << RATELIMIT_CALC_SHIFT;

    /*
     * We have computed basic pos_ratio above based on global situation. If
     * the bdi is over/under its share of dirty pages, we want to scale
     * pos_ratio further down/up. That is done by the following mechanism.
     */

    /*
     * bdi setpoint
     *
     *        f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
     *
     *                        x_intercept - bdi_dirty
     *                     := --------------------------
     *                        x_intercept - bdi_setpoint
     *
     * The main bdi control line is a linear function that subjects to
     *
     * (1) f(bdi_setpoint) = 1.0
     * (2) k = - 1 / (8 * write_bw)  (in single bdi case)
     *     or equally: x_intercept = bdi_setpoint + 8 * write_bw
     *
     * For single bdi case, the dirty pages are observed to fluctuate
     * regularly within range
     *        [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
     * for various filesystems, where (2) can yield in a reasonable 12.5%
     * fluctuation range for pos_ratio.
     *
     * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate up to its
     * own size, so move the slope over accordingly and choose a slope that
     * yields 100% pos_ratio fluctuation on suddenly doubled bdi_thresh.
     */
    if (unlikely(bdi_thresh > thresh))
        bdi_thresh = thresh;
    /*
     * It's very possible that bdi_thresh is close to 0 not because the
     * device is slow, but that it has remained inactive for long time.
     * Honour such devices a reasonable good (hopefully IO efficient)
     * threshold, so that the occasional writes won't be blocked and active
     * writes can rampup the threshold quickly.
     */
    bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
    /*
     * scale global setpoint to bdi's:
     *  bdi_setpoint = setpoint * bdi_thresh / thresh
     */
    x = div_u64((u64)bdi_thresh << 16, thresh + 1);
    bdi_setpoint = setpoint * (u64)x >> 16;
    /*
     * Use span=(8*write_bw) in single bdi case as indicated by
     * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD case.
     *
     *        bdi_thresh                    thresh - bdi_thresh
     * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh
     *          thresh                            thresh
     */
    span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
    x_intercept = bdi_setpoint + span;

    if (bdi_dirty < x_intercept - span / 4) {
        pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
                    x_intercept - bdi_setpoint + 1);
    } else
        pos_ratio /= 4;

    /*
     * bdi reserve area, safeguard against dirty pool underrun and disk idle
     * It may push the desired control point of global dirty pages higher
     * than setpoint.
     */
    x_intercept = bdi_thresh / 2;
    if (bdi_dirty < x_intercept) {
        if (bdi_dirty > x_intercept / 8)
            pos_ratio = div_u64(pos_ratio * x_intercept, bdi_dirty);
        else
            pos_ratio *= 8;
    }

    return pos_ratio;
}

static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
                       unsigned long elapsed,
                       unsigned long written)
{
    const unsigned long period = roundup_pow_of_two(3 * HZ);
    unsigned long avg = bdi->avg_write_bandwidth;
    unsigned long old = bdi->write_bandwidth;
    u64 bw;

    /*
     * bw = written * HZ / elapsed
     *
     *                   bw * elapsed + write_bandwidth * (period - elapsed)
     * write_bandwidth = ---------------------------------------------------
     *                                          period
     */
    bw = written - bdi->written_stamp;
    bw *= HZ;
    if (unlikely(elapsed > period)) {
        do_div(bw, elapsed);
        avg = bw;
        goto out;
    }
    bw += (u64)bdi->write_bandwidth * (period - elapsed);
    bw >>= ilog2(period);

    /*
     * one more level of smoothing, for filtering out sudden spikes
     */
    if (avg > old && old >= (unsigned long)bw)
        avg -= (avg - old) >> 3;

    if (avg < old && old <= (unsigned long)bw)
        avg += (old - avg) >> 3;

out:
    bdi->write_bandwidth = bw;
    bdi->avg_write_bandwidth = avg;
}

/*
 * The global dirtyable memory and dirty threshold could be suddenly knocked
 * down by a large amount (eg. on the startup of KVM in a swapless system).
 * This may throw the system into deep dirty exceeded state and throttle
 * heavy/light dirtiers alike. To retain good responsiveness, maintain
 * global_dirty_limit for tracking slowly down to the knocked down dirty
 * threshold.
 */
static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
{
    unsigned long limit = global_dirty_limit;

    /*
     * Follow up in one step.
     */
    if (limit < thresh) {
        limit = thresh;
        goto update;
    }

    /*
     * Follow down slowly. Use the higher one as the target, because thresh
     * may drop below dirty. This is exactly the reason to introduce
     * global_dirty_limit which is guaranteed to lie above the dirty pages.
     */
    thresh = max(thresh, dirty);
    if (limit > thresh) {
        limit -= (limit - thresh) >> 5;
        goto update;
    }
    return;
update:
    global_dirty_limit = limit;
}

static void global_update_bandwidth(unsigned long thresh,
                    unsigned long dirty,
                    unsigned long now)
{
    static DEFINE_SPINLOCK(dirty_lock);
    static unsigned long update_time;

    /*
     * check locklessly first to optimize away locking for the most time
     */
    if (time_before(now, update_time + BANDWIDTH_INTERVAL))
        return;

    spin_lock(&dirty_lock);
    if (time_after_eq(now, update_time + BANDWIDTH_INTERVAL)) {
        update_dirty_limit(thresh, dirty);
        update_time = now;
    }
    spin_unlock(&dirty_lock);
}

/*
 * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.
 *
 * Normal bdi tasks will be curbed at or below it in long term.
 * Obviously it should be around (write_bw / N) when there are N dd tasks.
 */
static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
                       unsigned long thresh,
                       unsigned long bg_thresh,
                       unsigned long dirty,
                       unsigned long bdi_thresh,
                       unsigned long bdi_dirty,
                       unsigned long dirtied,
                       unsigned long elapsed)
{
    unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
    unsigned long limit = hard_dirty_limit(thresh);
    unsigned long setpoint = (freerun + limit) / 2;
    unsigned long write_bw = bdi->avg_write_bandwidth;
    unsigned long dirty_ratelimit = bdi->dirty_ratelimit;
    unsigned long dirty_rate;
    unsigned long task_ratelimit;
    unsigned long balanced_dirty_ratelimit;
    unsigned long pos_ratio;
    unsigned long step;
    unsigned long x;

    /*
     * The dirty rate will match the writeout rate in long term, except
     * when dirty pages are truncated by userspace or re-dirtied by FS.
     */
    dirty_rate = (dirtied - bdi->dirtied_stamp) * HZ / elapsed;

    pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty,
                       bdi_thresh, bdi_dirty);
    /*
     * task_ratelimit reflects each dd's dirty rate for the past 200ms.
     */
    task_ratelimit = (u64)dirty_ratelimit *
                    pos_ratio >> RATELIMIT_CALC_SHIFT;
    task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */

    /*
     * A linear estimation of the "balanced" throttle rate. The theory is,
     * if there are N dd tasks, each throttled at task_ratelimit, the bdi's
     * dirty_rate will be measured to be (N * task_ratelimit). So the below
     * formula will yield the balanced rate limit (write_bw / N).
     *
     * Note that the expanded form is not a pure rate feedback:
     *  rate_(i+1) = rate_(i) * (write_bw / dirty_rate)          (1)
     * but also takes pos_ratio into account:
     *  rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
     *
     * (1) is not realistic because pos_ratio also takes part in balancing
     * the dirty rate.  Consider the state
     *  pos_ratio = 0.5                          (3)
     *  rate = 2 * (write_bw / N)                    (4)
     * If (1) is used, it will stuck in that state! Because each dd will
     * be throttled at
     *  task_ratelimit = pos_ratio * rate = (write_bw / N)       (5)
     * yielding
     *  dirty_rate = N * task_ratelimit = write_bw           (6)
     * put (6) into (1) we get
     *  rate_(i+1) = rate_(i)                        (7)
     *
     * So we end up using (2) to always keep
     *  rate_(i+1) ~= (write_bw / N)                     (8)
     * regardless of the value of pos_ratio. As long as (8) is satisfied,
     * pos_ratio is able to drive itself to 1.0, which is not only where
     * the dirty count meet the setpoint, but also where the slope of
     * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
     */
    balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
                       dirty_rate | 1);
    /*
     * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
     */
    if (unlikely(balanced_dirty_ratelimit > write_bw))
        balanced_dirty_ratelimit = write_bw;

    /*
     * We could safely do this and return immediately:
     *
     *  bdi->dirty_ratelimit = balanced_dirty_ratelimit;
     *
     * However to get a more stable dirty_ratelimit, the below elaborated
     * code makes use of task_ratelimit to filter out singular points and
     * limit the step size.
     *
     * The below code essentially only uses the relative value of
     *
     *  task_ratelimit - dirty_ratelimit
     *  = (pos_ratio - 1) * dirty_ratelimit
     *
     * which reflects the direction and size of dirty position error.
     */

    /*
     * dirty_ratelimit will follow balanced_dirty_ratelimit iff
     * task_ratelimit is on the same side of dirty_ratelimit, too.
     * For example, when
     * - dirty_ratelimit > balanced_dirty_ratelimit
     * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
     * lowering dirty_ratelimit will help meet both the position and rate
     * control targets. Otherwise, don't update dirty_ratelimit if it will
     * only help meet the rate target. After all, what the users ultimately
     * feel and care are stable dirty rate and small position error.
     *
     * |task_ratelimit - dirty_ratelimit| is used to limit the step size
     * and filter out the singular points of balanced_dirty_ratelimit. Which
     * keeps jumping around randomly and can even leap far away at times
     * due to the small 200ms estimation period of dirty_rate (we want to
     * keep that period small to reduce time lags).
     */
    step = 0;
    if (dirty < setpoint) {
        x = min(bdi->balanced_dirty_ratelimit,
             min(balanced_dirty_ratelimit, task_ratelimit));
        if (dirty_ratelimit < x)
            step = x - dirty_ratelimit;
    } else {
        x = max(bdi->balanced_dirty_ratelimit,
             max(balanced_dirty_ratelimit, task_ratelimit));
        if (dirty_ratelimit > x)
            step = dirty_ratelimit - x;
    }

    /*
     * Don't pursue 100% rate matching. It's impossible since the balanced
     * rate itself is constantly fluctuating. So decrease the track speed
     * when it gets close to the target. Helps eliminate pointless tremors.
     */
    step >>= dirty_ratelimit / (2 * step + 1);
    /*
     * Limit the tracking speed to avoid overshooting.
     */
    step = (step + 7) / 8;

    if (dirty_ratelimit < balanced_dirty_ratelimit)
        dirty_ratelimit += step;
    else
        dirty_ratelimit -= step;

    bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
    bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;

    trace_bdi_dirty_ratelimit(bdi, dirty_rate, task_ratelimit);
}

void __bdi_update_bandwidth(struct backing_dev_info *bdi,
                unsigned long thresh,
                unsigned long bg_thresh,
                unsigned long dirty,
                unsigned long bdi_thresh,
                unsigned long bdi_dirty,
                unsigned long start_time)
{
    unsigned long now = jiffies;
    unsigned long elapsed = now - bdi->bw_time_stamp;
    unsigned long dirtied;
    unsigned long written;

    /*
     * rate-limit, only update once every 200ms.
     */
    if (elapsed < BANDWIDTH_INTERVAL)
        return;

    dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]);
    written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);

    /*
     * Skip quiet periods when disk bandwidth is under-utilized.
     * (at least 1s idle time between two flusher runs)
     */
    if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
        goto snapshot;

    if (thresh) {
        global_update_bandwidth(thresh, dirty, now);
        bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty,
                       bdi_thresh, bdi_dirty,
                       dirtied, elapsed);
    }
    bdi_update_write_bandwidth(bdi, elapsed, written);

snapshot:
    bdi->dirtied_stamp = dirtied;
    bdi->written_stamp = written;
    bdi->bw_time_stamp = now;
}

static void bdi_update_bandwidth(struct backing_dev_info *bdi,
                 unsigned long thresh,
                 unsigned long bg_thresh,
                 unsigned long dirty,
                 unsigned long bdi_thresh,
                 unsigned long bdi_dirty,
                 unsigned long start_time)
{
    if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
        return;
    spin_lock(&bdi->wb.list_lock);
    __bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
                   bdi_thresh, bdi_dirty, start_time);
    spin_unlock(&bdi->wb.list_lock);
}

/*
 * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr()
 * will look to see if it needs to start dirty throttling.
 *
 * If dirty_poll_interval is too low, big NUMA machines will call the expensive
 * global_page_state() too often. So scale it near-sqrt to the safety margin
 * (the number of pages we may dirty without exceeding the dirty limits).
 */
static unsigned long dirty_poll_interval(unsigned long dirty,
                     unsigned long thresh)
{
    if (thresh > dirty)
        return 1UL << (ilog2(thresh - dirty) >> 1);

    return 1;
}

static long bdi_max_pause(struct backing_dev_info *bdi,
              unsigned long bdi_dirty)
{
    long bw = bdi->avg_write_bandwidth;
    long t;

    /*
     * Limit pause time for small memory systems. If sleeping for too long
     * time, a small pool of dirty/writeback pages may go empty and disk go
     * idle.
     *
     * 8 serves as the safety ratio.
     */
    t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
    t++;

    return min_t(long, t, MAX_PAUSE);
}

static long bdi_min_pause(struct backing_dev_info *bdi,
              long max_pause,
              unsigned long task_ratelimit,
              unsigned long dirty_ratelimit,
              int *nr_dirtied_pause)
{
    long hi = ilog2(bdi->avg_write_bandwidth);
    long lo = ilog2(bdi->dirty_ratelimit);
    long t;     /* target pause */
    long pause; /* estimated next pause */
    int pages;  /* target nr_dirtied_pause */

    /* target for 10ms pause on 1-dd case */
    t = max(1, HZ / 100);

    /*
     * Scale up pause time for concurrent dirtiers in order to reduce CPU
     * overheads.
     *
     * (N * 10ms) on 2^N concurrent tasks.
     */
    if (hi > lo)
        t += (hi - lo) * (10 * HZ) / 1024;

    /*
     * This is a bit convoluted. We try to base the next nr_dirtied_pause
     * on the much more stable dirty_ratelimit. However the next pause time
     * will be computed based on task_ratelimit and the two rate limits may
     * depart considerably at some time. Especially if task_ratelimit goes
     * below dirty_ratelimit/2 and the target pause is max_pause, the next
     * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
     * result task_ratelimit won't be executed faithfully, which could
     * eventually bring down dirty_ratelimit.
     *
     * We apply two rules to fix it up:
     * 1) try to estimate the next pause time and if necessary, use a lower
     *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
     *    nr_dirtied_pause will be "dancing" with task_ratelimit.
     * 2) limit the target pause time to max_pause/2, so that the normal
     *    small fluctuations of task_ratelimit won't trigger rule (1) and
     *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
     */
    t = min(t, 1 + max_pause / 2);
    pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);

    /*
     * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
     * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
     * When the 16 consecutive reads are often interrupted by some dirty
     * throttling pause during the async writes, cfq will go into idles
     * (deadline is fine). So push nr_dirtied_pause as high as possible
     * until reaches DIRTY_POLL_THRESH=32 pages.
     */
    if (pages < DIRTY_POLL_THRESH) {
        t = max_pause;
        pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
        if (pages > DIRTY_POLL_THRESH) {
            pages = DIRTY_POLL_THRESH;
            t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
        }
    }

    pause = HZ * pages / (task_ratelimit + 1);
    if (pause > max_pause) {
        t = max_pause;
        pages = task_ratelimit * t / roundup_pow_of_two(HZ);
    }

    *nr_dirtied_pause = pages;
    /*
     * The minimal pause time will normally be half the target pause time.
     */
    return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
}

/*
 * balance_dirty_pages() must be called by processes which are generating dirty
 * data.  It looks at the number of dirty pages in the machine and will force
 * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
 * If we're over `background_thresh' then the writeback threads are woken to
 * perform some writeout.
 */
static void balance_dirty_pages(struct address_space *mapping,
                unsigned long pages_dirtied)
{
    unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
    unsigned long bdi_reclaimable;
    unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
    unsigned long bdi_dirty;
    unsigned long freerun;
    unsigned long background_thresh;
    unsigned long dirty_thresh;
    unsigned long bdi_thresh;
    long period;
    long pause;
    long max_pause;
    long min_pause;
    int nr_dirtied_pause;
    bool dirty_exceeded = false;
    unsigned long task_ratelimit;
    unsigned long dirty_ratelimit;
    unsigned long pos_ratio;
    struct backing_dev_info *bdi = mapping->backing_dev_info;
    unsigned long start_time = jiffies;

    for (;;) {
        unsigned long now = jiffies;

        /*
         * Unstable writes are a feature of certain networked
         * filesystems (i.e. NFS) in which data may have been
         * written to the server's write cache, but has not yet
         * been flushed to permanent storage.
         */
        nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
                    global_page_state(NR_UNSTABLE_NFS);
        nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);

        global_dirty_limits(&background_thresh, &dirty_thresh);

        /*
         * Throttle it only when the background writeback cannot
         * catch-up. This avoids (excessively) small writeouts
         * when the bdi limits are ramping up.
         */
        freerun = dirty_freerun_ceiling(dirty_thresh,
                        background_thresh);
        if (nr_dirty <= freerun) {
            current->dirty_paused_when = now;
            current->nr_dirtied = 0;
            current->nr_dirtied_pause =
                dirty_poll_interval(nr_dirty, dirty_thresh);
            break;
        }

        if (unlikely(!writeback_in_progress(bdi)))
            bdi_start_background_writeback(bdi);

        /*
         * bdi_thresh is not treated as some limiting factor as
         * dirty_thresh, due to reasons
         * - in JBOD setup, bdi_thresh can fluctuate a lot
         * - in a system with HDD and USB key, the USB key may somehow
         *   go into state (bdi_dirty >> bdi_thresh) either because
         *   bdi_dirty starts high, or because bdi_thresh drops low.
         *   In this case we don't want to hard throttle the USB key
         *   dirtiers for 100 seconds until bdi_dirty drops under
         *   bdi_thresh. Instead the auxiliary bdi control line in
         *   bdi_position_ratio() will let the dirtier task progress
         *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.
         */
        bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);

        /*
         * In order to avoid the stacked BDI deadlock we need
         * to ensure we accurately count the 'dirty' pages when
         * the threshold is low.
         *
         * Otherwise it would be possible to get thresh+n pages
         * reported dirty, even though there are thresh-m pages
         * actually dirty; with m+n sitting in the percpu
         * deltas.
         */
        if (bdi_thresh < 2 * bdi_stat_error(bdi)) {
            bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
            bdi_dirty = bdi_reclaimable +
                    bdi_stat_sum(bdi, BDI_WRITEBACK);
        } else {
            bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
            bdi_dirty = bdi_reclaimable +
                    bdi_stat(bdi, BDI_WRITEBACK);
        }

        dirty_exceeded = (bdi_dirty > bdi_thresh) &&
                  (nr_dirty > dirty_thresh);
        if (dirty_exceeded && !bdi->dirty_exceeded)
            bdi->dirty_exceeded = 1;

        bdi_update_bandwidth(bdi, dirty_thresh, background_thresh,
                     nr_dirty, bdi_thresh, bdi_dirty,
                     start_time);

        dirty_ratelimit = bdi->dirty_ratelimit;
        pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
                           background_thresh, nr_dirty,
                           bdi_thresh, bdi_dirty);
        task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
                            RATELIMIT_CALC_SHIFT;
        max_pause = bdi_max_pause(bdi, bdi_dirty);
        min_pause = bdi_min_pause(bdi, max_pause,
                      task_ratelimit, dirty_ratelimit,
                      &nr_dirtied_pause);

        if (unlikely(task_ratelimit == 0)) {
            period = max_pause;
            pause = max_pause;
            goto pause;
        }
        period = HZ * pages_dirtied / task_ratelimit;
        pause = period;
        if (current->dirty_paused_when)
            pause -= now - current->dirty_paused_when;
        /*
         * For less than 1s think time (ext3/4 may block the dirtier
         * for up to 800ms from time to time on 1-HDD; so does xfs,
         * however at much less frequency), try to compensate it in
         * future periods by updating the virtual time; otherwise just
         * do a reset, as it may be a light dirtier.
         */
        if (pause < min_pause) {
            trace_balance_dirty_pages(bdi,
                          dirty_thresh,
                          background_thresh,
                          nr_dirty,
                          bdi_thresh,
                          bdi_dirty,
                          dirty_ratelimit,
                          task_ratelimit,
                          pages_dirtied,
                          period,
                          min(pause, 0L),
                          start_time);
            if (pause < -HZ) {
                current->dirty_paused_when = now;
                current->nr_dirtied = 0;
            } else if (period) {
                current->dirty_paused_when += period;
                current->nr_dirtied = 0;
            } else if (current->nr_dirtied_pause <= pages_dirtied)
                current->nr_dirtied_pause += pages_dirtied;
            break;
        }
        if (unlikely(pause > max_pause)) {
            /* for occasional dropped task_ratelimit */
            now += min(pause - max_pause, max_pause);
            pause = max_pause;
        }

pause:
        trace_balance_dirty_pages(bdi,
                      dirty_thresh,
                      background_thresh,
                      nr_dirty,
                      bdi_thresh,
                      bdi_dirty,
                      dirty_ratelimit,
                      task_ratelimit,
                      pages_dirtied,
                      period,
                      pause,
                      start_time);
        __set_current_state(TASK_KILLABLE);
        io_schedule_timeout(pause);

        current->dirty_paused_when = now + pause;
        current->nr_dirtied = 0;
        current->nr_dirtied_pause = nr_dirtied_pause;

        /*
         * This is typically equal to (nr_dirty < dirty_thresh) and can
         * also keep "1000+ dd on a slow USB stick" under control.
         */
        if (task_ratelimit)
            break;

        /*
         * In the case of an unresponding NFS server and the NFS dirty
         * pages exceeds dirty_thresh, give the other good bdi's a pipe
         * to go through, so that tasks on them still remain responsive.
         *
         * In theory 1 page is enough to keep the comsumer-producer
         * pipe going: the flusher cleans 1 page => the task dirties 1
         * more page. However bdi_dirty has accounting errors.  So use
         * the larger and more IO friendly bdi_stat_error.
         */
        if (bdi_dirty <= bdi_stat_error(bdi))
            break;

        if (fatal_signal_pending(current))
            break;
    }

    if (!dirty_exceeded && bdi->dirty_exceeded)
        bdi->dirty_exceeded = 0;

    if (writeback_in_progress(bdi))
        return;

    /*
     * In laptop mode, we wait until hitting the higher threshold before
     * starting background writeout, and then write out all the way down
     * to the lower threshold.  So slow writers cause minimal disk activity.
     *
     * In normal mode, we start background writeout at the lower
     * background_thresh, to keep the amount of dirty memory low.
     */
    if (laptop_mode)
        return;

    if (nr_reclaimable > background_thresh)
        bdi_start_background_writeback(bdi);
}

void set_page_dirty_balance(struct page *page, int page_mkwrite)
{
    if (set_page_dirty(page) || page_mkwrite) {
        struct address_space *mapping = page_mapping(page);

        if (mapping)
            balance_dirty_pages_ratelimited(mapping);
    }
}

static DEFINE_PER_CPU(int, bdp_ratelimits);

/*
 * Normal tasks are throttled by
 *  loop {
 *      dirty tsk->nr_dirtied_pause pages;
 *      take a snap in balance_dirty_pages();
 *  }
 * However there is a worst case. If every task exit immediately when dirtied
 * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
 * called to throttle the page dirties. The solution is to save the not yet
 * throttled page dirties in dirty_throttle_leaks on task exit and charge them
 * randomly into the running tasks. This works well for the above worst case,
 * as the new task will pick up and accumulate the old task's leaked dirty
 * count and eventually get throttled.
 */
DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;

void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
                    unsigned long nr_pages_dirtied)
{
    struct backing_dev_info *bdi = mapping->backing_dev_info;
    int ratelimit;
    int *p;

    if (!bdi_cap_account_dirty(bdi))
        return;

    ratelimit = current->nr_dirtied_pause;
    if (bdi->dirty_exceeded)
        ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));

    preempt_disable();
    /*
     * This prevents one CPU to accumulate too many dirtied pages without
     * calling into balance_dirty_pages(), which can happen when there are
     * 1000+ tasks, all of them start dirtying pages at exactly the same
     * time, hence all honoured too large initial task->nr_dirtied_pause.
     */
    p =  &__get_cpu_var(bdp_ratelimits);
    if (unlikely(current->nr_dirtied >= ratelimit))
        *p = 0;
    else if (unlikely(*p >= ratelimit_pages)) {
        *p = 0;
        ratelimit = 0;
    }
    /*
     * Pick up the dirtied pages by the exited tasks. This avoids lots of
     * short-lived tasks (eg. gcc invocations in a kernel build) escaping
     * the dirty throttling and livelock other long-run dirtiers.
     */
    p = &__get_cpu_var(dirty_throttle_leaks);
    if (*p > 0 && current->nr_dirtied < ratelimit) {
        nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
        *p -= nr_pages_dirtied;
        current->nr_dirtied += nr_pages_dirtied;
    }
    preempt_enable();

    if (unlikely(current->nr_dirtied >= ratelimit))
        balance_dirty_pages(mapping, current->nr_dirtied);
}
EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);

void throttle_vm_writeout(gfp_t gfp_mask)
{
    unsigned long background_thresh;
    unsigned long dirty_thresh;

        for ( ; ; ) {
        global_dirty_limits(&background_thresh, &dirty_thresh);
        dirty_thresh = hard_dirty_limit(dirty_thresh);

                /*
                 * Boost the allowable dirty threshold a bit for page
                 * allocators so they don't get DoS'ed by heavy writers
                 */
                dirty_thresh += dirty_thresh / 10;      /* wheeee... */

                if (global_page_state(NR_UNSTABLE_NFS) +
            global_page_state(NR_WRITEBACK) <= dirty_thresh)
                            break;
                congestion_wait(BLK_RW_ASYNC, HZ/10);

        /*
         * The caller might hold locks which can prevent IO completion
         * or progress in the filesystem.  So we cannot just sit here
         * waiting for IO to complete.
         */
        if ((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO))
            break;
        }
}

/*
 * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
 */
int dirty_writeback_centisecs_handler(ctl_table *table, int write,
    void __user *buffer, size_t *length, loff_t *ppos)
{
    proc_dointvec(table, write, buffer, length, ppos);
    return 0;
}

#ifdef CONFIG_BLOCK
void laptop_mode_timer_fn(unsigned long data)
{
    struct request_queue *q = (struct request_queue *)data;
    int nr_pages = global_page_state(NR_FILE_DIRTY) +
        global_page_state(NR_UNSTABLE_NFS);

    /*
     * We want to write everything out, not just down to the dirty
     * threshold
     */
    if (bdi_has_dirty_io(&q->backing_dev_info))
        bdi_start_writeback(&q->backing_dev_info, nr_pages,
                    WB_REASON_LAPTOP_TIMER);
}

/*
 * We've spun up the disk and we're in laptop mode: schedule writeback
 * of all dirty data a few seconds from now.  If the flush is already scheduled
 * then push it back - the user is still using the disk.
 */
void laptop_io_completion(struct backing_dev_info *info)
{
    mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
}

/*
 * We're in laptop mode and we've just synced. The sync's writes will have
 * caused another writeback to be scheduled by laptop_io_completion.
 * Nothing needs to be written back anymore, so we unschedule the writeback.
 */
void laptop_sync_completion(void)
{
    struct backing_dev_info *bdi;

    rcu_read_lock();

    list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
        del_timer(&bdi->laptop_mode_wb_timer);

    rcu_read_unlock();
}
#endif

/*
 * If ratelimit_pages is too high then we can get into dirty-data overload
 * if a large number of processes all perform writes at the same time.
 * If it is too low then SMP machines will call the (expensive)
 * get_writeback_state too often.
 *
 * Here we set ratelimit_pages to a level which ensures that when all CPUs are
 * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
 * thresholds.
 */

void writeback_set_ratelimit(void)
{
    unsigned long background_thresh;
    unsigned long dirty_thresh;
    global_dirty_limits(&background_thresh, &dirty_thresh);
    global_dirty_limit = dirty_thresh;
    ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
    if (ratelimit_pages < 16)
        ratelimit_pages = 16;
}

static int __cpuinit
ratelimit_handler(struct notifier_block *self, unsigned long action,
          void *hcpu)
{

    switch (action & ~CPU_TASKS_FROZEN) {
    case CPU_ONLINE:
    case CPU_DEAD:
        writeback_set_ratelimit();
        return NOTIFY_OK;
    default:
        return NOTIFY_DONE;
    }
}

static struct notifier_block __cpuinitdata ratelimit_nb = {
    .notifier_call  = ratelimit_handler,
    .next       = NULL,
};

/*
 * Called early on to tune the page writeback dirty limits.
 *
 * We used to scale dirty pages according to how total memory
 * related to pages that could be allocated for buffers (by
 * comparing nr_free_buffer_pages() to vm_total_pages.
 *
 * However, that was when we used "dirty_ratio" to scale with
 * all memory, and we don't do that any more. "dirty_ratio"
 * is now applied to total non-HIGHPAGE memory (by subtracting
 * totalhigh_pages from vm_total_pages), and as such we can't
 * get into the old insane situation any more where we had
 * large amounts of dirty pages compared to a small amount of
 * non-HIGHMEM memory.
 *
 * But we might still want to scale the dirty_ratio by how
 * much memory the box has..
 */
void __init page_writeback_init(void)
{
    writeback_set_ratelimit();
    register_cpu_notifier(&ratelimit_nb);

    fprop_global_init(&writeout_completions);
}

/*
 * We tag pages in batches of WRITEBACK_TAG_BATCH to reduce tree_lock latency.
 */
void tag_pages_for_writeback(struct address_space *mapping,
                 pgoff_t start, pgoff_t end)
{
#define WRITEBACK_TAG_BATCH 4096
    unsigned long tagged;

    do {
        spin_lock_irq(&mapping->tree_lock);
        tagged = radix_tree_range_tag_if_tagged(&mapping->page_tree,
                &start, end, WRITEBACK_TAG_BATCH,
                PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
        spin_unlock_irq(&mapping->tree_lock);
        WARN_ON_ONCE(tagged > WRITEBACK_TAG_BATCH);
        cond_resched();
        /* We check 'start' to handle wrapping when end == ~0UL */
    } while (tagged >= WRITEBACK_TAG_BATCH && start);
}
EXPORT_SYMBOL(tag_pages_for_writeback);

int write_cache_pages(struct address_space *mapping,
              struct writeback_control *wbc, writepage_t writepage,
              void *data)
{
    int ret = 0;
    int done = 0;
    struct pagevec pvec;
    int nr_pages;
    pgoff_t uninitialized_var(writeback_index);
    pgoff_t index;
    pgoff_t end;        /* Inclusive */
    pgoff_t done_index;
    int cycled;
    int range_whole = 0;
    int tag;

    pagevec_init(&pvec, 0);
    if (wbc->range_cyclic) {
        writeback_index = mapping->writeback_index; /* prev offset */
        index = writeback_index;
        if (index == 0)
            cycled = 1;
        else
            cycled = 0;
        end = -1;
    } else {
        index = wbc->range_start >> PAGE_CACHE_SHIFT;
        end = wbc->range_end >> PAGE_CACHE_SHIFT;
        if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
            range_whole = 1;
        cycled = 1; /* ignore range_cyclic tests */
    }
    if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
        tag = PAGECACHE_TAG_TOWRITE;
    else
        tag = PAGECACHE_TAG_DIRTY;
retry:
    if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
        tag_pages_for_writeback(mapping, index, end);
    done_index = index;
    while (!done && (index <= end)) {
        int i;

        nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
                  min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
        if (nr_pages == 0)
            break;

        for (i = 0; i < nr_pages; i++) {
            struct page *page = pvec.pages[i];

            /*
             * At this point, the page may be truncated or
             * invalidated (changing page->mapping to NULL), or
             * even swizzled back from swapper_space to tmpfs file
             * mapping. However, page->index will not change
             * because we have a reference on the page.
             */
            if (page->index > end) {
                /*
                 * can't be range_cyclic (1st pass) because
                 * end == -1 in that case.
                 */
                done = 1;
                break;
            }

            done_index = page->index;

            lock_page(page);

            /*
             * Page truncated or invalidated. We can freely skip it
             * then, even for data integrity operations: the page
             * has disappeared concurrently, so there could be no
             * real expectation of this data interity operation
             * even if there is now a new, dirty page at the same
             * pagecache address.
             */
            if (unlikely(page->mapping != mapping)) {
continue_unlock:
                unlock_page(page);
                continue;
            }

            if (!PageDirty(page)) {
                /* someone wrote it for us */
                goto continue_unlock;
            }

            if (PageWriteback(page)) {
                if (wbc->sync_mode != WB_SYNC_NONE)
                    wait_on_page_writeback(page);
                else
                    goto continue_unlock;
            }

            BUG_ON(PageWriteback(page));
            if (!clear_page_dirty_for_io(page))
                goto continue_unlock;

            trace_wbc_writepage(wbc, mapping->backing_dev_info);
            ret = (*writepage)(page, wbc, data);
            if (unlikely(ret)) {
                if (ret == AOP_WRITEPAGE_ACTIVATE) {
                    unlock_page(page);
                    ret = 0;
                } else {
                    /*
                     * done_index is set past this page,
                     * so media errors will not choke
                     * background writeout for the entire
                     * file. This has consequences for
                     * range_cyclic semantics (ie. it may
                     * not be suitable for data integrity
                     * writeout).
                     */
                    done_index = page->index + 1;
                    done = 1;
                    break;
                }
            }

            /*
             * We stop writing back only if we are not doing
             * integrity sync. In case of integrity sync we have to
             * keep going until we have written all the pages
             * we tagged for writeback prior to entering this loop.
             */
            if (--wbc->nr_to_write <= 0 &&
                wbc->sync_mode == WB_SYNC_NONE) {
                done = 1;
                break;
            }
        }
        pagevec_release(&pvec);
        cond_resched();
    }
    if (!cycled && !done) {
        /*
         * range_cyclic:
         * We hit the last page and there is more work to be done: wrap
         * back to the start of the file
         */
        cycled = 1;
        index = 0;
        end = writeback_index - 1;
        goto retry;
    }
    if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
        mapping->writeback_index = done_index;

    return ret;
}
EXPORT_SYMBOL(write_cache_pages);

/*
 * Function used by generic_writepages to call the real writepage
 * function and set the mapping flags on error
 */
static int __writepage(struct page *page, struct writeback_control *wbc,
               void *data)
{
    struct address_space *mapping = data;
    int ret = mapping->a_ops->writepage(page, wbc);
    mapping_set_error(mapping, ret);
    return ret;
}

int generic_writepages(struct address_space *mapping,
               struct writeback_control *wbc)
{
    struct blk_plug plug;
    int ret;

    /* deal with chardevs and other special file */
    if (!mapping->a_ops->writepage)
        return 0;

    blk_start_plug(&plug);
    ret = write_cache_pages(mapping, wbc, __writepage, mapping);
    blk_finish_plug(&plug);
    return ret;
}

EXPORT_SYMBOL(generic_writepages);

int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
{
    int ret;

    if (wbc->nr_to_write <= 0)
        return 0;
    if (mapping->a_ops->writepages)
        ret = mapping->a_ops->writepages(mapping, wbc);
    else
        ret = generic_writepages(mapping, wbc);
    return ret;
}

int write_one_page(struct page *page, int wait)
{
    struct address_space *mapping = page->mapping;
    int ret = 0;
    struct writeback_control wbc = {
        .sync_mode = WB_SYNC_ALL,
        .nr_to_write = 1,
    };

    BUG_ON(!PageLocked(page));

    if (wait)
        wait_on_page_writeback(page);

    if (clear_page_dirty_for_io(page)) {
        page_cache_get(page);
        ret = mapping->a_ops->writepage(page, &wbc);
        if (ret == 0 && wait) {
            wait_on_page_writeback(page);
            if (PageError(page))
                ret = -EIO;
        }
        page_cache_release(page);
    } else {
        unlock_page(page);
    }
    return ret;
}
EXPORT_SYMBOL(write_one_page);

/*
 * For address_spaces which do not use buffers nor write back.
 */
int __set_page_dirty_no_writeback(struct page *page)
{
    if (!PageDirty(page))
        return !TestSetPageDirty(page);
    return 0;
}

/*
 * Helper function for set_page_dirty family.
 * NOTE: This relies on being atomic wrt interrupts.
 */
void account_page_dirtied(struct page *page, struct address_space *mapping)
{
    if (mapping_cap_account_dirty(mapping)) {
        __inc_zone_page_state(page, NR_FILE_DIRTY);
        __inc_zone_page_state(page, NR_DIRTIED);
        __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
        __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
        task_io_account_write(PAGE_CACHE_SIZE);
        current->nr_dirtied++;
        this_cpu_inc(bdp_ratelimits);
    }
}
EXPORT_SYMBOL(account_page_dirtied);

/*
 * Helper function for set_page_writeback family.
 * NOTE: Unlike account_page_dirtied this does not rely on being atomic
 * wrt interrupts.
 */
void account_page_writeback(struct page *page)
{
    inc_zone_page_state(page, NR_WRITEBACK);
}
EXPORT_SYMBOL(account_page_writeback);

/*
 * For address_spaces which do not use buffers.  Just tag the page as dirty in
 * its radix tree.
 *
 * This is also used when a single buffer is being dirtied: we want to set the
 * page dirty in that case, but not all the buffers.  This is a "bottom-up"
 * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
 *
 * Most callers have locked the page, which pins the address_space in memory.
 * But zap_pte_range() does not lock the page, however in that case the
 * mapping is pinned by the vma's ->vm_file reference.
 *
 * We take care to handle the case where the page was truncated from the
 * mapping by re-checking page_mapping() inside tree_lock.
 */
int __set_page_dirty_nobuffers(struct page *page)
{
    if (!TestSetPageDirty(page)) {
        struct address_space *mapping = page_mapping(page);
        struct address_space *mapping2;

        if (!mapping)
            return 1;

        spin_lock_irq(&mapping->tree_lock);
        mapping2 = page_mapping(page);
        if (mapping2) { /* Race with truncate? */
            BUG_ON(mapping2 != mapping);
            WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
            account_page_dirtied(page, mapping);
            radix_tree_tag_set(&mapping->page_tree,
                page_index(page), PAGECACHE_TAG_DIRTY);
        }
        spin_unlock_irq(&mapping->tree_lock);
        if (mapping->host) {
            /* !PageAnon && !swapper_space */
            __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
        }
        return 1;
    }
    return 0;
}
EXPORT_SYMBOL(__set_page_dirty_nobuffers);

/*
 * Call this whenever redirtying a page, to de-account the dirty counters
 * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
 * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
 * systematic errors in balanced_dirty_ratelimit and the dirty pages position
 * control.
 */
void account_page_redirty(struct page *page)
{
    struct address_space *mapping = page->mapping;
    if (mapping && mapping_cap_account_dirty(mapping)) {
        current->nr_dirtied--;
        dec_zone_page_state(page, NR_DIRTIED);
        dec_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
    }
}
EXPORT_SYMBOL(account_page_redirty);

/*
 * When a writepage implementation decides that it doesn't want to write this
 * page for some reason, it should redirty the locked page via
 * redirty_page_for_writepage() and it should then unlock the page and return 0
 */
int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
{
    wbc->pages_skipped++;
    account_page_redirty(page);
    return __set_page_dirty_nobuffers(page);
}
EXPORT_SYMBOL(redirty_page_for_writepage);

/*
 * Dirty a page.
 *
 * For pages with a mapping this should be done under the page lock
 * for the benefit of asynchronous memory errors who prefer a consistent
 * dirty state. This rule can be broken in some special cases,
 * but should be better not to.
 *
 * If the mapping doesn't provide a set_page_dirty a_op, then
 * just fall through and assume that it wants buffer_heads.
 */
int set_page_dirty(struct page *page)
{
    struct address_space *mapping = page_mapping(page);

    if (likely(mapping)) {
        int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
        /*
         * readahead/lru_deactivate_page could remain
         * PG_readahead/PG_reclaim due to race with end_page_writeback
         * About readahead, if the page is written, the flags would be
         * reset. So no problem.
         * About lru_deactivate_page, if the page is redirty, the flag
         * will be reset. So no problem. but if the page is used by readahead
         * it will confuse readahead and make it restart the size rampup
         * process. But it's a trivial problem.
         */
        ClearPageReclaim(page);
#ifdef CONFIG_BLOCK
        if (!spd)
            spd = __set_page_dirty_buffers;
#endif
        return (*spd)(page);
    }
    if (!PageDirty(page)) {
        if (!TestSetPageDirty(page))
            return 1;
    }
    return 0;
}
EXPORT_SYMBOL(set_page_dirty);

/*
 * set_page_dirty() is racy if the caller has no reference against
 * page->mapping->host, and if the page is unlocked.  This is because another
 * CPU could truncate the page off the mapping and then free the mapping.
 *
 * Usually, the page _is_ locked, or the caller is a user-space process which
 * holds a reference on the inode by having an open file.
 *
 * In other cases, the page should be locked before running set_page_dirty().
 */
int set_page_dirty_lock(struct page *page)
{
    int ret;

    lock_page(page);
    ret = set_page_dirty(page);
    unlock_page(page);
    return ret;
}
EXPORT_SYMBOL(set_page_dirty_lock);

/*
 * Clear a page's dirty flag, while caring for dirty memory accounting.
 * Returns true if the page was previously dirty.
 *
 * This is for preparing to put the page under writeout.  We leave the page
 * tagged as dirty in the radix tree so that a concurrent write-for-sync
 * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
 * implementation will run either set_page_writeback() or set_page_dirty(),
 * at which stage we bring the page's dirty flag and radix-tree dirty tag
 * back into sync.
 *
 * This incoherency between the page's dirty flag and radix-tree tag is
 * unfortunate, but it only exists while the page is locked.
 */
int clear_page_dirty_for_io(struct page *page)
{
    struct address_space *mapping = page_mapping(page);

    BUG_ON(!PageLocked(page));

    if (mapping && mapping_cap_account_dirty(mapping)) {
        /*
         * Yes, Virginia, this is indeed insane.
         *
         * We use this sequence to make sure that
         *  (a) we account for dirty stats properly
         *  (b) we tell the low-level filesystem to
         *      mark the whole page dirty if it was
         *      dirty in a pagetable. Only to then
         *  (c) clean the page again and return 1 to
         *      cause the writeback.
         *
         * This way we avoid all nasty races with the
         * dirty bit in multiple places and clearing
         * them concurrently from different threads.
         *
         * Note! Normally the "set_page_dirty(page)"
         * has no effect on the actual dirty bit - since
         * that will already usually be set. But we
         * need the side effects, and it can help us
         * avoid races.
         *
         * We basically use the page "master dirty bit"
         * as a serialization point for all the different
         * threads doing their things.
         */
        if (page_mkclean(page))
            set_page_dirty(page);
        /*
         * We carefully synchronise fault handlers against
         * installing a dirty pte and marking the page dirty
         * at this point. We do this by having them hold the
         * page lock at some point after installing their
         * pte, but before marking the page dirty.
         * Pages are always locked coming in here, so we get
         * the desired exclusion. See mm/memory.c:do_wp_page()
         * for more comments.
         */
        if (TestClearPageDirty(page)) {
            dec_zone_page_state(page, NR_FILE_DIRTY);
            dec_bdi_stat(mapping->backing_dev_info,
                    BDI_RECLAIMABLE);
            return 1;
        }
        return 0;
    }
    return TestClearPageDirty(page);
}
EXPORT_SYMBOL(clear_page_dirty_for_io);

int test_clear_page_writeback(struct page *page)
{
    struct address_space *mapping = page_mapping(page);
    int ret;

    if (mapping) {
        struct backing_dev_info *bdi = mapping->backing_dev_info;
        unsigned long flags;

        spin_lock_irqsave(&mapping->tree_lock, flags);
        ret = TestClearPageWriteback(page);
        if (ret) {
            radix_tree_tag_clear(&mapping->page_tree,
                        page_index(page),
                        PAGECACHE_TAG_WRITEBACK);
            if (bdi_cap_account_writeback(bdi)) {
                __dec_bdi_stat(bdi, BDI_WRITEBACK);
                __bdi_writeout_inc(bdi);
            }
        }
        spin_unlock_irqrestore(&mapping->tree_lock, flags);
    } else {
        ret = TestClearPageWriteback(page);
    }
    if (ret) {
        dec_zone_page_state(page, NR_WRITEBACK);
        inc_zone_page_state(page, NR_WRITTEN);
    }
    return ret;
}

int test_set_page_writeback(struct page *page)
{
    struct address_space *mapping = page_mapping(page);
    int ret;

    if (mapping) {
        struct backing_dev_info *bdi = mapping->backing_dev_info;
        unsigned long flags;

        spin_lock_irqsave(&mapping->tree_lock, flags);
        ret = TestSetPageWriteback(page);
        if (!ret) {
            radix_tree_tag_set(&mapping->page_tree,
                        page_index(page),
                        PAGECACHE_TAG_WRITEBACK);
            if (bdi_cap_account_writeback(bdi))
                __inc_bdi_stat(bdi, BDI_WRITEBACK);
        }
        if (!PageDirty(page))
            radix_tree_tag_clear(&mapping->page_tree,
                        page_index(page),
                        PAGECACHE_TAG_DIRTY);
        radix_tree_tag_clear(&mapping->page_tree,
                     page_index(page),
                     PAGECACHE_TAG_TOWRITE);
        spin_unlock_irqrestore(&mapping->tree_lock, flags);
    } else {
        ret = TestSetPageWriteback(page);
    }
    if (!ret)
        account_page_writeback(page);
    return ret;

}
EXPORT_SYMBOL(test_set_page_writeback);

/*
 * Return true if any of the pages in the mapping are marked with the
 * passed tag.
 */
int mapping_tagged(struct address_space *mapping, int tag)
{
    return radix_tree_tagged(&mapping->page_tree, tag);
}
EXPORT_SYMBOL(mapping_tagged);