[PATCH 3.12 067/146] mm/page-writeback.c: fix divide by zero in pos_ratio_polynom

Jiri Slaby
2014-06-09T09:27:39Z

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

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

===============

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@redhat.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
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 7106cb1aca8e..8f6daa62206d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -593,14 +593,14 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
* (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
* => fast response on large errors; small oscillation near setpoint
*/
-static inline long long pos_ratio_polynom(unsigned long setpoint,
+static long long pos_ratio_polynom(unsigned long setpoint,
unsigned long dirty,
unsigned long limit)
{
long long pos_ratio;
long x;

- 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;
@@ -842,7 +842,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
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;
--
1.9.3