writeback: set max_pause to lowest value on zero bdi_dirty

Some trace shows lots of bdi_dirty=0 lines where it's actually some
small value if w/o the accounting errors in the per-cpu bdi stats.

In this case the max pause time should really be set to the smallest
(non-zero) value to avoid IO queue underrun and improve throughput.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 17403e3a7c89..50f08241f981 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -989,8 +989,7 @@ static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
          *
          * 8 serves as the safety ratio.
          */
-        if (bdi_dirty)
-                t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+        t = min(t, bdi_dirty * HZ / (8 * bw + 1));
 
         /*
          * The pause time will be settled within range (max_pause/4, max_pause).