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Commit 3a2e9a5a authored by Wu Fengguang's avatar Wu Fengguang Committed by Jens Axboe
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writeback: balance_dirty_pages() shall write more than dirtied pages 



Some filesystem may choose to write much more than ratelimit_pages
before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
determine number to write based on real number of dirtied pages.

Otherwise it is possible that
  loop {
    btrfs_file_write():     dirty 1024 pages
    balance_dirty_pages():  write up to 48 pages (= ratelimit_pages * 1.5)
  }
in which the writeback rate cannot keep up with dirty rate, and the
dirty pages go all the way beyond dirty_thresh.

The increased write_chunk may make the dirtier more bumpy.
So filesystems shall be take care not to dirty too much at
a time (eg. > 4MB) without checking the ratelimit.

Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent a5989bdc

mm/page-writeback.c

+10 −6
Original line number Diff line number Diff line
@@ -44,12 +44,15 @@ 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

 * 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(void)

static inline long sync_writeback_pages(unsigned long dirtied)

{

	return ratelimit_pages + ratelimit_pages / 2;

	if (dirtied < ratelimit_pages)

		dirtied = ratelimit_pages;



	return dirtied + dirtied / 2;

}



/* The following parameters are exported via /proc/sys/vm */
@@ -477,7 +480,8 @@ get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty, 
 * If we're over `background_thresh' then pdflush is woken to perform some

 * writeout.

 */

static void balance_dirty_pages(struct address_space *mapping)

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;
@@ -485,7 +489,6 @@ static void balance_dirty_pages(struct address_space *mapping) 
	unsigned long dirty_thresh;

	unsigned long bdi_thresh;

	unsigned long pages_written = 0;

	unsigned long write_chunk = sync_writeback_pages();

	unsigned long pause = 1;



	struct backing_dev_info *bdi = mapping->backing_dev_info;
@@ -640,9 +643,10 @@ void balance_dirty_pages_ratelimited_nr(struct address_space *mapping, 
	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);

		balance_dirty_pages(mapping, ratelimit);

		return;

	}

	preempt_enable();