[Devel] [PATCH RHEL7 COMMIT] ms/mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-229.7.2.vz7.8.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-229.7.2.vz7.8.3
------>
commit d693573172a1ce54f7eb7773967cb7eae30d41c2
Author: Maxim Patlasov <MPatlasov at parallels.com>
Date:   Wed Oct 7 15:27:55 2015 +0400

    ms/mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()
    
    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>
    
    https://jira.sw.ru/browse/PSBM-40116
    
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 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