spelling fixes: mm/

[safe/jmp/linux-2.6] / 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    akpm@zip.com.au
 *              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>

/*
 * The maximum number of pages to writeout in a single bdflush/kupdate
 * operation.  We do this so we don't hold I_SYNC against an inode for
 * enormous amounts of time, which would block a userspace task which has
 * been forced to throttle against that inode.  Also, the code reevaluates
 * the dirty each time it has written this many pages.
 */
#define MAX_WRITEBACK_PAGES     1024

/*
 * 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;

/*
 * When balance_dirty_pages decides that the caller needs to perform some
 * non-background writeback, this is how many pages it will attempt to write.
 * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably
 * large amounts of I/O are submitted.
 */
static inline long sync_writeback_pages(void)
{
        return ratelimit_pages + ratelimit_pages / 2;
}

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

/*
 * Start background writeback (via pdflush) at this percentage
 */
int dirty_background_ratio = 5;

/*
 * The generator of dirty data starts writeback at this percentage
 */
int vm_dirty_ratio = 10;

/*
 * The interval between `kupdate'-style writebacks, in jiffies
 */
int dirty_writeback_interval = 5 * HZ;

/*
 * The longest number of jiffies for which data is allowed to remain dirty
 */
int dirty_expire_interval = 30 * HZ;

/*
 * 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 */


static void background_writeout(unsigned long _min_pages);

/*
 * 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;

static unsigned long determine_dirtyable_memory(void);

/*
 * 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;

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

/*
 * update the period when the dirty ratio changes.
 */
int dirty_ratio_handler(struct ctl_table *table, int write,
                struct file *filp, void __user *buffer, size_t *lenp,
                loff_t *ppos)
{
        int old_ratio = vm_dirty_ratio;
        int ret = proc_dointvec_minmax(table, write, filp, buffer, lenp, ppos);
        if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
                int shift = calc_period_shift();
                prop_change_shift(&vm_completions, shift);
                prop_change_shift(&vm_dirties, shift);
        }
        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)
{
        __prop_inc_percpu(&vm_completions, &bdi->completions);
}

static inline 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)
{
        if (bdi_cap_writeback_dirty(bdi)) {
                prop_fraction_percpu(&vm_completions, &bdi->completions,
                                numerator, denominator);
        } else {
                *numerator = 0;
                *denominator = 1;
        }
}

/*
 * Clip the earned share of dirty pages to that which is actually available.
 * This avoids exceeding the total dirty_limit when the floating averages
 * fluctuate too quickly.
 */
static void
clip_bdi_dirty_limit(struct backing_dev_info *bdi, long dirty, long *pbdi_dirty)
{
        long avail_dirty;

        avail_dirty = dirty -
                (global_page_state(NR_FILE_DIRTY) +
                 global_page_state(NR_WRITEBACK) +
                 global_page_state(NR_UNSTABLE_NFS));

        if (avail_dirty < 0)
                avail_dirty = 0;

        avail_dirty += bdi_stat(bdi, BDI_RECLAIMABLE) +
                bdi_stat(bdi, BDI_WRITEBACK);

        *pbdi_dirty = min(*pbdi_dirty, avail_dirty);
}

static inline void task_dirties_fraction(struct task_struct *tsk,
                long *numerator, long *denominator)
{
        prop_fraction_single(&vm_dirties, &tsk->dirties,
                                numerator, denominator);
}

/*
 * scale the dirty limit
 *
 * task specific dirty limit:
 *
 *   dirty -= (dirty/8) * p_{t}
 */
void task_dirty_limit(struct task_struct *tsk, long *pdirty)
{
        long numerator, denominator;
        long dirty = *pdirty;
        u64 inv = dirty >> 3;

        task_dirties_fraction(tsk, &numerator, &denominator);
        inv *= numerator;
        do_div(inv, denominator);

        dirty -= inv;
        if (dirty < *pdirty/2)
                dirty = *pdirty/2;

        *pdirty = dirty;
}

/*
 * 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_page_state(z, NR_INACTIVE)
                        + zone_page_state(z, NR_ACTIVE);
        }
        /*
         * 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 determine_dirtyable_memory(void)
{
        unsigned long x;

        x = global_page_state(NR_FREE_PAGES)
                + global_page_state(NR_INACTIVE)
                + global_page_state(NR_ACTIVE);
        x -= highmem_dirtyable_memory(x);
        return x + 1;   /* Ensure that we never return 0 */
}

static void
get_dirty_limits(long *pbackground, long *pdirty, long *pbdi_dirty,
                 struct backing_dev_info *bdi)
{
        int background_ratio;           /* Percentages */
        int dirty_ratio;
        int unmapped_ratio;
        long background;
        long dirty;
        unsigned long available_memory = determine_dirtyable_memory();
        struct task_struct *tsk;

        unmapped_ratio = 100 - ((global_page_state(NR_FILE_MAPPED) +
                                global_page_state(NR_ANON_PAGES)) * 100) /
                                        available_memory;

        dirty_ratio = vm_dirty_ratio;
        if (dirty_ratio > unmapped_ratio / 2)
                dirty_ratio = unmapped_ratio / 2;

        if (dirty_ratio < 5)
                dirty_ratio = 5;

        background_ratio = dirty_background_ratio;
        if (background_ratio >= dirty_ratio)
                background_ratio = dirty_ratio / 2;

        background = (background_ratio * available_memory) / 100;
        dirty = (dirty_ratio * available_memory) / 100;
        tsk = current;
        if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
                background += background / 4;
                dirty += dirty / 4;
        }
        *pbackground = background;
        *pdirty = dirty;

        if (bdi) {
                u64 bdi_dirty = dirty;
                long numerator, denominator;

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

                bdi_dirty *= numerator;
                do_div(bdi_dirty, denominator);

                *pbdi_dirty = bdi_dirty;
                clip_bdi_dirty_limit(bdi, dirty, pbdi_dirty);
                task_dirty_limit(current, pbdi_dirty);
        }
}

/*
 * 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 perform writeback if the system is over `vm_dirty_ratio'.
 * If we're over `background_thresh' then pdflush is woken to perform some
 * writeout.
 */
static void balance_dirty_pages(struct address_space *mapping)
{
        long bdi_nr_reclaimable;
        long bdi_nr_writeback;
        long background_thresh;
        long dirty_thresh;
        long bdi_thresh;
        unsigned long pages_written = 0;
        unsigned long write_chunk = sync_writeback_pages();

        struct backing_dev_info *bdi = mapping->backing_dev_info;

        for (;;) {
                struct writeback_control wbc = {
                        .bdi            = bdi,
                        .sync_mode      = WB_SYNC_NONE,
                        .older_than_this = NULL,
                        .nr_to_write    = write_chunk,
                        .range_cyclic   = 1,
                };

                get_dirty_limits(&background_thresh, &dirty_thresh,
                                &bdi_thresh, bdi);
                bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
                bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
                if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
                        break;

                if (!bdi->dirty_exceeded)
                        bdi->dirty_exceeded = 1;

                /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
                 * 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.
                 */
                if (bdi_nr_reclaimable) {
                        writeback_inodes(&wbc);
                        pages_written += write_chunk - wbc.nr_to_write;
                        get_dirty_limits(&background_thresh, &dirty_thresh,
                                       &bdi_thresh, bdi);
                }

                /*
                 * 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_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
                        bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
                } else if (bdi_nr_reclaimable) {
                        bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
                        bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
                }

                if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
                        break;
                if (pages_written >= write_chunk)
                        break;          /* We've done our duty */

                congestion_wait(WRITE, HZ/10);
        }

        if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
                        bdi->dirty_exceeded)
                bdi->dirty_exceeded = 0;

        if (writeback_in_progress(bdi))
                return;         /* pdflush is already working this queue */

        /*
         * 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 && pages_written) ||
                        (!laptop_mode && (global_page_state(NR_FILE_DIRTY)
                                          + global_page_state(NR_UNSTABLE_NFS)
                                          > background_thresh)))
                pdflush_operation(background_writeout, 0);
}

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);
        }
}

/**
 * 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)
{
        static DEFINE_PER_CPU(unsigned long, ratelimits) = 0;
        unsigned long ratelimit;
        unsigned long *p;

        ratelimit = ratelimit_pages;
        if (mapping->backing_dev_info->dirty_exceeded)
                ratelimit = 8;

        /*
         * Check the rate limiting. Also, we do not want to throttle real-time
         * tasks in balance_dirty_pages(). Period.
         */
        preempt_disable();
        p =  &__get_cpu_var(ratelimits);
        *p += nr_pages_dirtied;
        if (unlikely(*p >= ratelimit)) {
                *p = 0;
                preempt_enable();
                balance_dirty_pages(mapping);
                return;
        }
        preempt_enable();
}
EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);

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

        for ( ; ; ) {
                get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);

                /*
                 * 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(WRITE, 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;
        }
}

/*
 * writeback at least _min_pages, and keep writing until the amount of dirty
 * memory is less than the background threshold, or until we're all clean.
 */
static void background_writeout(unsigned long _min_pages)
{
        long min_pages = _min_pages;
        struct writeback_control wbc = {
                .bdi            = NULL,
                .sync_mode      = WB_SYNC_NONE,
                .older_than_this = NULL,
                .nr_to_write    = 0,
                .nonblocking    = 1,
                .range_cyclic   = 1,
        };

        for ( ; ; ) {
                long background_thresh;
                long dirty_thresh;

                get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
                if (global_page_state(NR_FILE_DIRTY) +
                        global_page_state(NR_UNSTABLE_NFS) < background_thresh
                                && min_pages <= 0)
                        break;
                wbc.more_io = 0;
                wbc.encountered_congestion = 0;
                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
                wbc.pages_skipped = 0;
                writeback_inodes(&wbc);
                min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
                if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
                        /* Wrote less than expected */
                        if (wbc.encountered_congestion || wbc.more_io)
                                congestion_wait(WRITE, HZ/10);
                        else
                                break;
                }
        }
}

/*
 * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
 * the whole world.  Returns 0 if a pdflush thread was dispatched.  Returns
 * -1 if all pdflush threads were busy.
 */
int wakeup_pdflush(long nr_pages)
{
        if (nr_pages == 0)
                nr_pages = global_page_state(NR_FILE_DIRTY) +
                                global_page_state(NR_UNSTABLE_NFS);
        return pdflush_operation(background_writeout, nr_pages);
}

static void wb_timer_fn(unsigned long unused);
static void laptop_timer_fn(unsigned long unused);

static DEFINE_TIMER(wb_timer, wb_timer_fn, 0, 0);
static DEFINE_TIMER(laptop_mode_wb_timer, laptop_timer_fn, 0, 0);

/*
 * Periodic writeback of "old" data.
 *
 * Define "old": the first time one of an inode's pages is dirtied, we mark the
 * dirtying-time in the inode's address_space.  So this periodic writeback code
 * just walks the superblock inode list, writing back any inodes which are
 * older than a specific point in time.
 *
 * Try to run once per dirty_writeback_interval.  But if a writeback event
 * takes longer than a dirty_writeback_interval interval, then leave a
 * one-second gap.
 *
 * older_than_this takes precedence over nr_to_write.  So we'll only write back
 * all dirty pages if they are all attached to "old" mappings.
 */
static void wb_kupdate(unsigned long arg)
{
        unsigned long oldest_jif;
        unsigned long start_jif;
        unsigned long next_jif;
        long nr_to_write;
        struct writeback_control wbc = {
                .bdi            = NULL,
                .sync_mode      = WB_SYNC_NONE,
                .older_than_this = &oldest_jif,
                .nr_to_write    = 0,
                .nonblocking    = 1,
                .for_kupdate    = 1,
                .range_cyclic   = 1,
        };

        sync_supers();

        oldest_jif = jiffies - dirty_expire_interval;
        start_jif = jiffies;
        next_jif = start_jif + dirty_writeback_interval;
        nr_to_write = global_page_state(NR_FILE_DIRTY) +
                        global_page_state(NR_UNSTABLE_NFS) +
                        (inodes_stat.nr_inodes - inodes_stat.nr_unused);
        while (nr_to_write > 0) {
                wbc.more_io = 0;
                wbc.encountered_congestion = 0;
                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
                writeback_inodes(&wbc);
                if (wbc.nr_to_write > 0) {
                        if (wbc.encountered_congestion || wbc.more_io)
                                congestion_wait(WRITE, HZ/10);
                        else
                                break;  /* All the old data is written */
                }
                nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
        }
        if (time_before(next_jif, jiffies + HZ))
                next_jif = jiffies + HZ;
        if (dirty_writeback_interval)
                mod_timer(&wb_timer, next_jif);
}

/*
 * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
 */
int dirty_writeback_centisecs_handler(ctl_table *table, int write,
        struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
{
        proc_dointvec_userhz_jiffies(table, write, file, buffer, length, ppos);
        if (dirty_writeback_interval)
                mod_timer(&wb_timer, jiffies + dirty_writeback_interval);
        else
                del_timer(&wb_timer);
        return 0;
}

static void wb_timer_fn(unsigned long unused)
{
        if (pdflush_operation(wb_kupdate, 0) < 0)
                mod_timer(&wb_timer, jiffies + HZ); /* delay 1 second */
}

static void laptop_flush(unsigned long unused)
{
        sys_sync();
}

static void laptop_timer_fn(unsigned long unused)
{
        pdflush_operation(laptop_flush, 0);
}

/*
 * 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(void)
{
        mod_timer(&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)
{
        del_timer(&laptop_mode_wb_timer);
}

/*
 * 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 before writeback cuts in.
 *
 * But the limit should not be set too high.  Because it also controls the
 * amount of memory which the balance_dirty_pages() caller has to write back.
 * If this is too large then the caller will block on the IO queue all the
 * time.  So limit it to four megabytes - the balance_dirty_pages() caller
 * will write six megabyte chunks, max.
 */

void writeback_set_ratelimit(void)
{
        ratelimit_pages = vm_total_pages / (num_online_cpus() * 32);
        if (ratelimit_pages < 16)
                ratelimit_pages = 16;
        if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
                ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
}

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;

        mod_timer(&wb_timer, jiffies + dirty_writeback_interval);
        writeback_set_ratelimit();
        register_cpu_notifier(&ratelimit_nb);

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

/**
 * 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.
 */
int write_cache_pages(struct address_space *mapping,
                      struct writeback_control *wbc, writepage_t writepage,
                      void *data)
{
        struct backing_dev_info *bdi = mapping->backing_dev_info;
        int ret = 0;
        int done = 0;
        struct pagevec pvec;
        int nr_pages;
        pgoff_t index;
        pgoff_t end;            /* Inclusive */
        int scanned = 0;
        int range_whole = 0;

        if (wbc->nonblocking && bdi_write_congested(bdi)) {
                wbc->encountered_congestion = 1;
                return 0;
        }

        pagevec_init(&pvec, 0);
        if (wbc->range_cyclic) {
                index = mapping->writeback_index; /* Start from prev offset */
                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;
                scanned = 1;
        }
retry:
        while (!done && (index <= end) &&
               (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
                                              PAGECACHE_TAG_DIRTY,
                                              min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
                unsigned i;

                scanned = 1;
                for (i = 0; i < nr_pages; i++) {
                        struct page *page = pvec.pages[i];

                        /*
                         * At this point we hold neither mapping->tree_lock nor
                         * lock on the page itself: the page may be truncated or
                         * invalidated (changing page->mapping to NULL), or even
                         * swizzled back from swapper_space to tmpfs file
                         * mapping
                         */
                        lock_page(page);

                        if (unlikely(page->mapping != mapping)) {
                                unlock_page(page);
                                continue;
                        }

                        if (!wbc->range_cyclic && page->index > end) {
                                done = 1;
                                unlock_page(page);
                                continue;
                        }

                        if (wbc->sync_mode != WB_SYNC_NONE)
                                wait_on_page_writeback(page);

                        if (PageWriteback(page) ||
                            !clear_page_dirty_for_io(page)) {
                                unlock_page(page);
                                continue;
                        }

                        ret = (*writepage)(page, wbc, data);

                        if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) {
                                unlock_page(page);
                                ret = 0;
                        }
                        if (ret || (--(wbc->nr_to_write) <= 0))
                                done = 1;
                        if (wbc->nonblocking && bdi_write_congested(bdi)) {
                                wbc->encountered_congestion = 1;
                                done = 1;
                        }
                }
                pagevec_release(&pvec);
                cond_resched();
        }
        if (!scanned && !done) {
                /*
                 * We hit the last page and there is more work to be done: wrap
                 * back to the start of the file
                 */
                scanned = 1;
                index = 0;
                goto retry;
        }
        if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
                mapping->writeback_index = 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)
{
        /* deal with chardevs and other special file */
        if (!mapping->a_ops->writepage)
                return 0;

        return write_cache_pages(mapping, wbc, __writepage, mapping);
}

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;
        wbc->for_writepages = 1;
        if (mapping->a_ops->writepages)
                ret = mapping->a_ops->writepages(mapping, wbc);
        else
                ret = generic_writepages(mapping, wbc);
        wbc->for_writepages = 0;
        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))
                SetPageDirty(page);
        return 0;
}

/*
 * 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;

                write_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));
                        if (mapping_cap_account_dirty(mapping)) {
                                __inc_zone_page_state(page, NR_FILE_DIRTY);
                                __inc_bdi_stat(mapping->backing_dev_info,
                                                BDI_RECLAIMABLE);
                                task_io_account_write(PAGE_CACHE_SIZE);
                        }
                        radix_tree_tag_set(&mapping->page_tree,
                                page_index(page), PAGECACHE_TAG_DIRTY);
                }
                write_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);

/*
 * If the mapping doesn't provide a set_page_dirty a_op, then
 * just fall through and assume that it wants buffer_heads.
 */
static 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;
#ifdef CONFIG_BLOCK
                if (!spd)
                        spd = __set_page_dirty_buffers;
#endif
                return (*spd)(page);
        }
        if (!PageDirty(page)) {
                if (!TestSetPageDirty(page))
                        return 1;
        }
        return 0;
}

int fastcall set_page_dirty(struct page *page)
{
        int ret = __set_page_dirty(page);
        if (ret)
                task_dirty_inc(current);
        return ret;
}
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_nosync(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));

        ClearPageReclaim(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;

                write_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_writeback_dirty(bdi)) {
                                __dec_bdi_stat(bdi, BDI_WRITEBACK);
                                __bdi_writeout_inc(bdi);
                        }
                }
                write_unlock_irqrestore(&mapping->tree_lock, flags);
        } else {
                ret = TestClearPageWriteback(page);
        }
        if (ret)
                dec_zone_page_state(page, NR_WRITEBACK);
        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;

                write_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_writeback_dirty(bdi))
                                __inc_bdi_stat(bdi, BDI_WRITEBACK);
                }
                if (!PageDirty(page))
                        radix_tree_tag_clear(&mapping->page_tree,
                                                page_index(page),
                                                PAGECACHE_TAG_DIRTY);
                write_unlock_irqrestore(&mapping->tree_lock, flags);
        } else {
                ret = TestSetPageWriteback(page);
        }
        if (!ret)
                inc_zone_page_state(page, NR_WRITEBACK);
        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)
{
        int ret;
        rcu_read_lock();
        ret = radix_tree_tagged(&mapping->page_tree, tag);
        rcu_read_unlock();
        return ret;
}
EXPORT_SYMBOL(mapping_tagged);