writeback: avoid unnecessary calculation of bdi dirty thresholds

[firefly-linux-kernel-4.4.55.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>

/*
 * 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 dirtied pages to ensure that reasonably
 * large amounts of I/O are submitted.
 */
static inline long sync_writeback_pages(unsigned long dirtied)
{
        if (dirtied < ratelimit_pages)
                dirtied = ratelimit_pages;

        return dirtied + dirtied / 2;
}

/* 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 */


/*
 * 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);
}

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

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}
 */
static unsigned long task_dirty_limit(struct task_struct *tsk,
                                       unsigned long bdi_dirty)
{
        long numerator, denominator;
        unsigned long dirty = bdi_dirty;
        u64 inv = dirty >> 3;

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

        dirty -= inv;

        return max(dirty, bdi_dirty/2);
}

/*
 *
 */
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 */
}

void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
{
        unsigned long background;
        unsigned long dirty;
        unsigned long available_memory = determine_dirtyable_memory();
        struct task_struct *tsk;

        if (vm_dirty_bytes)
                dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
        else {
                int dirty_ratio;

                dirty_ratio = vm_dirty_ratio;
                if (dirty_ratio < 5)
                        dirty_ratio = 5;
                dirty = (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;
}

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;
}

/*
 * 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 the writeback threads are woken to
 * perform some writeout.
 */
static void balance_dirty_pages(struct address_space *mapping,
                                unsigned long write_chunk)
{
        long nr_reclaimable, bdi_nr_reclaimable;
        long nr_writeback, bdi_nr_writeback;
        unsigned long background_thresh;
        unsigned long dirty_thresh;
        unsigned long bdi_thresh;
        unsigned long pages_written = 0;
        unsigned long pause = 1;
        bool dirty_exceeded = false;
        struct backing_dev_info *bdi = mapping->backing_dev_info;

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

                nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
                                        global_page_state(NR_UNSTABLE_NFS);
                nr_writeback = 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.
                 */
                if (nr_reclaimable + nr_writeback <
                                (background_thresh + dirty_thresh) / 2)
                        break;

                bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
                bdi_thresh = task_dirty_limit(current, bdi_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_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
                        bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
                } else {
                        bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
                        bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
                }

                /*
                 * The bdi thresh is somehow "soft" limit derived from the
                 * global "hard" limit. The former helps to prevent heavy IO
                 * bdi or process from holding back light ones; The latter is
                 * the last resort safeguard.
                 */
                dirty_exceeded =
                        (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
                        || (nr_reclaimable + nr_writeback >= dirty_thresh);

                if (!dirty_exceeded)
                        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.
                 * Only move pages to writeback if this bdi is over its
                 * threshold otherwise wait until the disk writes catch
                 * up.
                 */
                trace_wbc_balance_dirty_start(&wbc, bdi);
                if (bdi_nr_reclaimable > bdi_thresh) {
                        writeback_inodes_wb(&bdi->wb, &wbc);
                        pages_written += write_chunk - wbc.nr_to_write;
                        trace_wbc_balance_dirty_written(&wbc, bdi);
                        if (pages_written >= write_chunk)
                                break;          /* We've done our duty */
                }
                trace_wbc_balance_dirty_wait(&wbc, bdi);
                __set_current_state(TASK_INTERRUPTIBLE);
                io_schedule_timeout(pause);

                /*
                 * Increase the delay for each loop, up to our previous
                 * default of taking a 100ms nap.
                 */
                pause <<= 1;
                if (pause > HZ / 10)
                        pause = HZ / 10;
        }

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

        if (writeback_in_progress(bdi))
                return;

        /*
         * In laptop mode, we wait until hitting the higher threshold before
         * starting background writeout, and then write out all the way down
         * to the lower threshold.  So slow writers cause minimal disk activity.
         *
         * In normal mode, we start background writeout at the lower
         * background_thresh, to keep the amount of dirty memory low.
         */
        if ((laptop_mode && pages_written) ||
            (!laptop_mode && (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(unsigned long, bdp_ratelimits) = 0;

/**
 * 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)
{
        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(bdp_ratelimits);
        *p += nr_pages_dirtied;
        if (unlikely(*p >= ratelimit)) {
                ratelimit = sync_writeback_pages(*p);
                *p = 0;
                preempt_enable();
                balance_dirty_pages(mapping, ratelimit);
                return;
        }
        preempt_enable();
}
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);
}

/*
 * 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 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;

        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();
        } while (tagged >= WRITEBACK_TAG_BATCH);
}
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)
                tag = PAGECACHE_TAG_TOWRITE;
        else
                tag = PAGECACHE_TAG_DIRTY;
retry:
        if (wbc->sync_mode == WB_SYNC_ALL)
                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 + 1;

                        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 = 1;
                                        break;
                                }
                        }

                        if (wbc->nr_to_write > 0) {
                                if (--wbc->nr_to_write == 0 &&
                                    wbc->sync_mode == WB_SYNC_NONE) {
                                        /*
                                         * We stop writing back only if we are
                                         * not doing integrity sync. In case of
                                         * integrity sync we have to keep going
                                         * because someone may be concurrently
                                         * dirtying pages, and we might have
                                         * synced a lot of newly appeared dirty
                                         * pages, but have not synced all of the
                                         * old dirty pages.
                                         */
                                        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)
{
        /* 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;
        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))
                SetPageDirty(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_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
                task_dirty_inc(current);
                task_io_account_write(PAGE_CACHE_SIZE);
        }
}

/*
 * 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;
#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_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;

                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);
        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)
                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);