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Commit 62a584fe authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe
Browse files

writeback: use higher precision calculation in domain_dirty_limits() 



As vm.dirty_[background_]bytes can't be applied verbatim to multiple
cgroup writeback domains, they get converted to percentages in
domain_dirty_limits() and applied the same way as
vm.dirty_[background]ratio.  However, if the specified bytes is lower
than 1% of available memory, the calculated ratios become zero and the
writeback domain gets throttled constantly.

Fix it by using per-PAGE_SIZE instead of percentage for ratio
calculations.  Also, the updated DIV_ROUND_UP() usages now should
yield 1/4096 (0.0244%) as the minimum ratio as long as the specified
bytes are above zero.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Reported-by: default avatarMiao Xie <miaoxie@huawei.com>
Link: http://lkml.kernel.org/g/57333E75.3080309@huawei.com


Cc: stable@vger.kernel.org # v4.2+
Fixes: 9fc3a43e ("writeback: separate out domain_dirty_limits()")
Reviewed-by: default avatarJan Kara <jack@suse.cz>

Adjusted comment based on Jan's suggestion.
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent b02b1fbd

mm/page-writeback.c

+12 −9
Original line number Diff line number Diff line
@@ -373,8 +373,9 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc) 
	struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);

	unsigned long bytes = vm_dirty_bytes;

	unsigned long bg_bytes = dirty_background_bytes;

	unsigned long ratio = vm_dirty_ratio;

	unsigned long bg_ratio = dirty_background_ratio;

	/* convert ratios to per-PAGE_SIZE for higher precision */

	unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;

	unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;

	unsigned long thresh;

	unsigned long bg_thresh;

	struct task_struct *tsk;
@@ -386,26 +387,28 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc) 
		/*

		 * The byte settings can't be applied directly to memcg

		 * domains.  Convert them to ratios by scaling against

		 * globally available memory.

		 * globally available memory.  As the ratios are in

		 * per-PAGE_SIZE, they can be obtained by dividing bytes by

		 * number of pages.

		 */

		if (bytes)

			ratio = min(DIV_ROUND_UP(bytes, PAGE_SIZE) * 100 /

				    global_avail, 100UL);

			ratio = min(DIV_ROUND_UP(bytes, global_avail),

				    PAGE_SIZE);

		if (bg_bytes)

			bg_ratio = min(DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100 /

				       global_avail, 100UL);

			bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),

				       PAGE_SIZE);

		bytes = bg_bytes = 0;

	}



	if (bytes)

		thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);

	else

		thresh = (ratio * available_memory) / 100;

		thresh = (ratio * available_memory) / PAGE_SIZE;



	if (bg_bytes)

		bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);

	else

		bg_thresh = (bg_ratio * available_memory) / 100;

		bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;



	if (bg_thresh >= thresh)

		bg_thresh = thresh / 2;