linux-kernel - [PATCH 3.14 10/39] mm/page-writeback.c: fix divide by zero in bdi_dirty

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Date:	Tue,  5 Aug 2014 11:13:47 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Maxim Patlasov <mpatlasov@...allels.com>,
	Rik van Riel <riel@...hat.com>,
	Michal Hocko <mhocko@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 3.14 10/39] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Maxim Patlasov <MPatlasov@...allels.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@...allels.com>
Acked-by: Rik van Riel <riel@...hat.com>
Cc: Michal Hocko <mhocko@...e.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@...el.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1324,9 +1324,9 @@ static inline void bdi_dirty_limits(stru
 	*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


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