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

[Ben Hutchings]

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

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

From: Rik van Riel <riel@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Nishanth Aravamudan <nacc@xxxxxxxxxxxxxxxxxx>
Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
Cc: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2:
 Adjust context - pos_ratio_polynom() is not a separate function]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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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