mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()

Under memory pressure, it is possible for dirty_thresh, calculated by
global_dirty_limits() in balance_dirty_pages(), to equal zero.  Then, if
strictlimit is true, bdi_dirty_limits() tries to resolve the proportion:

  bdi_bg_thresh : bdi_thresh = background_thresh : dirty_thresh

by dividing by zero.

Signed-off-by: Maxim Patlasov <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 518e2c3f4c75f0014a03817b4910cf4050a2f480..e0c943014eb74ce3d4cb4c021d6f896740386d6d 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1306,9 +1306,9 @@ static inline void bdi_dirty_limits(struct backing_dev_info *bdi,
        *bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
 
        if (bdi_bg_thresh)
-               *bdi_bg_thresh = div_u64((u64)*bdi_thresh *
-                                        background_thresh,
-                                        dirty_thresh);
+               *bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *
+                                                       background_thresh,
+                                                       dirty_thresh) : 0;
 
        /*
         * In order to avoid the stacked BDI deadlock we need