Re: [PATCH block/for-4.7-fixes] writeback: use higher precision calculation in domain_dirty_limits()

From: Jens Axboe
Date: Mon May 30 2016 - 10:55:18 EST

On 05/27/2016 12:34 PM, Tejun Heo wrote:
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>
Cc: stable@xxxxxxxxxxxxxxx # v4.2+
Fixes: 9fc3a43e1757 ("writeback: separate out domain_dirty_limits()")

Queued up for this series, with the minor comment tweak that Jan suggested.

Jens Axboe