On 04/30/2014 09:48 AM, Michal Hocko wrote:
On Wed 30-04-14 09:30:35, Rik van Riel wrote:
[...]
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@xxxxxxxxxx>
---
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 */
+
This is still prone to u64 -> s32 issue, isn't it?
What was the original problem anyway? Was it really setpoint > limit or
rather the overflow?
Good point. I guess we need these divisors to be "int" and
"unsigned int" respectively, since those are the arguments
given to div_s64 and div_u64.
Let me send a v3 right now.
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;
Same here.
+ 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;
--
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