linux-kernel - [PATCH 3.2 60/92] mm/page-writeback.c: fix divide by zero in pos_ratio

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Date:	Sat, 07 Jun 2014 02:26:29 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Michal Hocko" <mhocko@...e.cz>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Rik van Riel" <riel@...hat.com>,
	"Nishanth Aravamudan" <nacc@...ux.vnet.ibm.com>,
	"Masayoshi Mizuma" <m.mizuma@...fujitsu.com>,
	"Mel Gorman" <mgorman@...e.de>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Luiz Capitulino" <lcapitulino@...hat.com>
Subject: [PATCH 3.2 60/92] mm/page-writeback.c: fix divide by zero in
 pos_ratio_polynom

3.2.60-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Rik van Riel <riel@...hat.com>

commit d5c9fde3dae750889168807038243ff36431d276 upstream.

It is possible for "limit - setpoint + 1" to equal zero, after getting
truncated to a 32 bit variable, and resulting in a divide by zero error.

Using the fully 64 bit divide functions avoids this problem.  It also
will cause pos_ratio_polynom() to return the correct value when
(setpoint - limit) exceeds 2^32.

Also uninline pos_ratio_polynom, at Andrew's request.

Signed-off-by: Rik van Riel <riel@...hat.com>
Reviewed-by: Michal Hocko <mhocko@...e.cz>
Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
Cc: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2:
 Adjust context - pos_ratio_polynom() is not a separate function]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/page-writeback.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -559,7 +559,7 @@ static unsigned long bdi_position_ratio(
 	 *     => fast response on large errors; small oscillation near setpoint
 	 */
 	setpoint = (freerun + limit) / 2;
-	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
+	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
 		    limit - setpoint + 1);
 	pos_ratio = x;
 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
@@ -625,7 +625,7 @@ static unsigned long bdi_position_ratio(
 	x_intercept = bdi_setpoint + span;
 
 	if (bdi_dirty < x_intercept - span / 4) {
-		pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
+		pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
 				    x_intercept - bdi_setpoint + 1);
 	} else
 		pos_ratio /= 4;

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