writeback: Add a 'reason' to wb_writeback_work

[mirror_ubuntu-artful-kernel.git] / mm / page-writeback.c

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

#include <linux/kernel.h>
#include <linux/module.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>
#include <linux/pagevec.h>
#include <trace/events/writeback.h>

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

/*
 * 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 */

/*
 * 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 prop_descriptor vm_completions;
static struct prop_descriptor vm_dirties;

/*
 * couple the period to the dirty_ratio:
 *
 *   period/2 ~ roundup_pow_of_two(dirty limit)
 */
static int calc_period_shift(void)
{
        unsigned long dirty_total;

        if (vm_dirty_bytes)
                dirty_total = vm_dirty_bytes / PAGE_SIZE;
        else
                dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
                                100;
        return 2 + ilog2(dirty_total - 1);
}

/*
 * update the period when the dirty threshold changes.
 */
static void update_completion_period(void)
{
        int shift = calc_period_shift();
        prop_change_shift(&vm_completions, shift);
        prop_change_shift(&vm_dirties, shift);

        writeback_set_ratelimit();
}

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) {
                update_completion_period();
                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) {
                update_completion_period();
                vm_dirty_ratio = 0;
        }
        return ret;
}

/*
 * 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);
        __prop_inc_percpu_max(&vm_completions, &bdi->completions,
                              bdi->max_prop_frac);
}

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);

void task_dirty_inc(struct task_struct *tsk)
{
        prop_inc_single(&vm_dirties, &tsk->dirties);
}

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

/*
 *
 */
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 = (PROP_FRAC_BASE * max_ratio) / 100;
        }
        spin_unlock_bh(&bdi_lock);

        return ret;
}
EXPORT_SYMBOL(bdi_set_max_ratio);

/*
 * 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.
 */

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);
        }
        /*
         * 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
}

/**
 * determine_dirtyable_memory - amount of memory that may be used
 *
 * Returns the numebr of pages that can currently be freed and used
 * by the kernel for direct mappings.
 */
unsigned long determine_dirtyable_memory(void)
{
        unsigned long x;

        x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();

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

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

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);
}

/*
 * 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 = determine_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);
}

/**
 * bdi_dirty_limit - @bdi's share of dirty throttling threshold
 * @bdi: the backing_dev_info to query
 * @dirty: global dirty limit in pages
 *
 * Returns @bdi's dirty limit in pages. The term "dirty" in the context of
 * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
 * And the "limit" in the name is not seriously taken as hard limit in
 * balance_dirty_pages().
 *
 * It allocates high/low dirty limits to fast/slow devices, in order to prevent
 * - starving fast devices
 * - piling up dirty pages (that will take long time to sync) on slow devices
 *
 * The bdi's share of dirty limit will be adapting to its throughput and
 * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
 */
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;
        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);

        /*
         * 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 sigular 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 sigular 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 unsigned long bdi_max_pause(struct backing_dev_info *bdi,
                                   unsigned long bdi_dirty)
{
        unsigned long bw = bdi->avg_write_bandwidth;
        unsigned long hi = ilog2(bw);
        unsigned long lo = ilog2(bdi->dirty_ratelimit);
        unsigned long t;

        /* target for 20ms max pause on 1-dd case */
        t = HZ / 50;

        /*
         * Scale up pause time for concurrent dirtiers in order to reduce CPU
         * overheads.
         *
         * (N * 20ms) on 2^N concurrent tasks.
         */
        if (hi > lo)
                t += (hi - lo) * (20 * HZ) / 1024;

        /*
         * 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.
         */
        if (bdi_dirty)
                t = min(t, bdi_dirty * HZ / (8 * bw + 1));

        /*
         * The pause time will be settled within range (max_pause/4, max_pause).
         * Apply a minimal value of 4 to get a non-zero max_pause/4.
         */
        return clamp_val(t, 4, MAX_PAUSE);
}

/*
 * 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 pause = 0;
        long uninitialized_var(max_pause);
        bool dirty_exceeded = false;
        unsigned long task_ratelimit;
        unsigned long uninitialized_var(dirty_ratelimit);
        unsigned long pos_ratio;
        struct backing_dev_info *bdi = mapping->backing_dev_info;
        unsigned long start_time = jiffies;

        for (;;) {
                /*
                 * 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)
                        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);

                max_pause = bdi_max_pause(bdi, bdi_dirty);

                dirty_ratelimit = bdi->dirty_ratelimit;
                pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
                                               background_thresh, nr_dirty,
                                               bdi_thresh, bdi_dirty);
                if (unlikely(pos_ratio == 0)) {
                        pause = max_pause;
                        goto pause;
                }
                task_ratelimit = (u64)dirty_ratelimit *
                                        pos_ratio >> RATELIMIT_CALC_SHIFT;
                pause = (HZ * pages_dirtied) / (task_ratelimit | 1);
                if (unlikely(pause <= 0)) {
                        trace_balance_dirty_pages(bdi,
                                                  dirty_thresh,
                                                  background_thresh,
                                                  nr_dirty,
                                                  bdi_thresh,
                                                  bdi_dirty,
                                                  dirty_ratelimit,
                                                  task_ratelimit,
                                                  pages_dirtied,
                                                  pause,
                                                  start_time);
                        pause = 1; /* avoid resetting nr_dirtied_pause below */
                        break;
                }
                pause = min(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,
                                          pause,
                                          start_time);
                __set_current_state(TASK_UNINTERRUPTIBLE);
                io_schedule_timeout(pause);

                dirty_thresh = hard_dirty_limit(dirty_thresh);
                /*
                 * max-pause area. If dirty exceeded but still within this
                 * area, no need to sleep for more than 200ms: (a) 8 pages per
                 * 200ms is typically more than enough to curb heavy dirtiers;
                 * (b) the pause time limit makes the dirtiers more responsive.
                 */
                if (nr_dirty < dirty_thresh)
                        break;
        }

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

        current->nr_dirtied = 0;
        if (pause == 0) { /* in freerun area */
                current->nr_dirtied_pause =
                                dirty_poll_interval(nr_dirty, dirty_thresh);
        } else if (pause <= max_pause / 4 &&
                   pages_dirtied >= current->nr_dirtied_pause) {
                current->nr_dirtied_pause = clamp_val(
                                        dirty_ratelimit * (max_pause / 2) / HZ,
                                        pages_dirtied + pages_dirtied / 8,
                                        pages_dirtied * 4);
        } else if (pause >= max_pause) {
                current->nr_dirtied_pause = 1 | clamp_val(
                                        dirty_ratelimit * (max_pause / 2) / HZ,
                                        pages_dirtied / 4,
                                        pages_dirtied - pages_dirtied / 8);
        }

        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);

/**
 * balance_dirty_pages_ratelimited_nr - balance dirty memory state
 * @mapping: address_space which was dirtied
 * @nr_pages_dirtied: number of pages which the caller has just dirtied
 *
 * Processes which are dirtying memory should call in here once for each page
 * which was newly dirtied.  The function will periodically check the system's
 * dirty state and will initiate writeback if needed.
 *
 * On really big machines, get_writeback_state is expensive, so try to avoid
 * calling it too often (ratelimiting).  But once we're over the dirty memory
 * limit we decrease the ratelimiting by a lot, to prevent individual processes
 * from overshooting the limit by (ratelimit_pages) each.
 */
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));

        current->nr_dirtied += nr_pages_dirtied;

        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 {
                *p += nr_pages_dirtied;
                if (unlikely(*p >= ratelimit_pages)) {
                        *p = 0;
                        ratelimit = 0;
                }
        }
        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);

                /*
                 * 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);
        bdi_arm_supers_timer();
        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);
        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 u, void *v)
{
        writeback_set_ratelimit();
        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)
{
        int shift;

        writeback_set_ratelimit();
        register_cpu_notifier(&ratelimit_nb);

        shift = calc_period_shift();
        prop_descriptor_init(&vm_completions, shift);
        prop_descriptor_init(&vm_dirties, shift);
}

/**
 * tag_pages_for_writeback - tag pages to be written by write_cache_pages
 * @mapping: address space structure to write
 * @start: starting page index
 * @end: ending page index (inclusive)
 *
 * This function scans the page range from @start to @end (inclusive) and tags
 * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
 * that write_cache_pages (or whoever calls this function) will then use
 * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
 * used to avoid livelocking of writeback by a process steadily creating new
 * dirty pages in the file (thus it is important for this function to be quick
 * so that it can tag pages faster than a dirtying process can create them).
 */
/*
 * 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);

/**
 * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
 * @mapping: address space structure to write
 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 * @writepage: function called for each page
 * @data: data passed to writepage function
 *
 * If a page is already under I/O, write_cache_pages() skips it, even
 * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 * and msync() need to guarantee that all the data which was dirty at the time
 * the call was made get new I/O started against them.  If wbc->sync_mode is
 * WB_SYNC_ALL then we were called for data integrity and we must wait for
 * existing IO to complete.
 *
 * To avoid livelocks (when other process dirties new pages), we first tag
 * pages which should be written back with TOWRITE tag and only then start
 * writing them. For data-integrity sync we have to be careful so that we do
 * not miss some pages (e.g., because some other process has cleared TOWRITE
 * tag we set). The rule we follow is that TOWRITE tag can be cleared only
 * by the process clearing the DIRTY tag (and submitting the page for IO).
 */
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;
}

/**
 * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
 * @mapping: address space structure to write
 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 *
 * This is a library function, which implements the writepages()
 * address_space_operation.
 */
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;
}

/**
 * write_one_page - write out a single page and optionally wait on I/O
 * @page: the page to write
 * @wait: if true, wait on writeout
 *
 * The page must be locked by the caller and will be unlocked upon return.
 *
 * write_one_page() returns a negative error code if I/O failed.
 */
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_dirty_inc(current);
                task_io_account_write(PAGE_CACHE_SIZE);
        }
}
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);

/*
 * 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++;
        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);