[PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

Rik van Riel
2014-04-30T13:44:34Z

On Wed, 30 Apr 2014 19:01:11 +0900
Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> wrote:

> Hi Rik,
>
> I applied your patch to linux-next kernel, then divide error happened
> when I ran ltp stress test.
> The divide error occurred on the following div_u64(), so the following
> should be also fixed...
>
> static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,

Good catch. This patch fixes both places, and also has Andrew's
improvements in both places.

Andrew, this can drop in -mm instead of my previous patch and
your two cleanups to it.

---8<---

Subject: mm,writeback: fix divide by zero in pos_ratio_polynom

It is possible for "limit - setpoint + 1" to equal zero, leading to a
divide by zero error. Blindly adding 1 to "limit - setpoint" is not
working, so we need to actually test the divisor before calling div64.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
mm/page-writeback.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ef41349..f98a297 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -597,11 +597,16 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
unsigned long dirty,
unsigned long limit)
{
+ unsigned long divisor;
long long pos_ratio;
long x;

+ divisor = limit - setpoint;
+ if (!divisor)
+ divisor = 1; /* Avoid div-by-zero */
+
x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
- limit - setpoint + 1);
+ divisor);
pos_ratio = x;
pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
@@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
x_intercept = bdi_setpoint + span;

if (bdi_dirty < x_intercept - span / 4) {
+ unsigned long divisor = x_intercept - bdi_setpoint;
+ if (!divisor)
+ divisor = 1; /* Avoid div-by-zero */
+
pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
- x_intercept - bdi_setpoint + 1);
+ divisor);
} else
pos_ratio /= 4;