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 <tj@kernel.org>
Reported-by: Miao Xie <miaoxie@huawei.com>
Link: http://lkml.kernel.org/g/57333E75.3080309@huawei.com
Cc: stable@vger.kernel.org # v4.2+
Fixes: 9fc3a43e1757 ("writeback: separate out domain_dirty_limits()")
Reviewed-by: Jan Kara <jack@suse.cz>
Adjusted comment based on Jan's suggestion.
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b9956fdee8f5d0ed55713eef51e13a6ea42b2e0d..e2481949494c4e6bf4cd0fd0799d178bd2203008 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -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;