4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Tejun Heo <tj@xxxxxxxxxx> commit 62a584fe05eef1f80ed49a286a29328f1a224fb9 upstream. 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: Tejun Heo <tj@xxxxxxxxxx> Reported-by: Miao Xie <miaoxie@xxxxxxxxxx> Link: http://lkml.kernel.org/g/57333E75.3080309@xxxxxxxxxx Fixes: 9fc3a43e1757 ("writeback: separate out domain_dirty_limits()") Reviewed-by: Jan Kara <jack@xxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Adjusted comment based on Jan's suggestion. Signed-off-by: Jens Axboe <axboe@xxxxxx> --- mm/page-writeback.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -359,8 +359,9 @@ static void domain_dirty_limits(struct d 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; @@ -372,26 +373,28 @@ static void domain_dirty_limits(struct d /* * 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; -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html