[Devel] [PATCH rh7 v2] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()

[Andrey Ryabinin]

From: Maxim Patlasov <MPatlasov at parallels.com>

commit f6789593d5cea42a4ecb1cbeab6a23ade5ebbba7 upstream.

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 <mpatlasov at parallels.com>
Acked-by: Rik van Riel <riel at redhat.com>
Cc: Michal Hocko <mhocko at suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu at intel.com>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

Fixes: https://jira.sw.ru/browse/PSBM-40116
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
Change since v1:
 Added reference to the upstream commit into changelog

 mm/page-writeback.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2bb28ba..429c759 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1366,9 +1366,9 @@ 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
-- 
2.4.9