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

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 <>
Reported-by: default avatarMiao Xie <>
Cc: # v4.2+
Fixes: 9fc3a43e ("writeback: separate out domain_dirty_limits()")
Reviewed-by: default avatarJan Kara <>

Adjusted comment based on Jan's suggestion.
Signed-off-by: default avatarJens Axboe <>
parent b02b1fbd
......@@ -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),
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),
bytes = bg_bytes = 0;
if (bytes)
thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
thresh = (ratio * available_memory) / 100;
thresh = (ratio * available_memory) / PAGE_SIZE;
if (bg_bytes)
bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
bg_thresh = (bg_ratio * available_memory) / 100;
bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
if (bg_thresh >= thresh)
bg_thresh = thresh / 2;
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