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: Tejun Heo <[email protected]>
Reported-by: Miao Xie <[email protected]>
Link: http://lkml.kernel.org/g/[email protected]
Cc: [email protected] # v4.2+
Fixes: 9fc3a43e1757 ("writeback: separate out domain_dirty_limits()")
Reviewed-by: Jan Kara <[email protected]>

Adjusted comment based on Jan's suggestion.
Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions