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>

1 files changed, 12 insertions, 9 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b9956fdee8f5..e2481949494c 100644
--- 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;