Messages in this thread Patch in this message | | | Date | Wed, 30 Apr 2014 10:41:14 -0400 | From | Rik van Riel <> | Subject | [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On Wed, 30 Apr 2014 15:48:26 +0200 Michal Hocko <mhocko@suse.cz> wrote:
> 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?
This patch should avoid math overflows with both the initial subtraction, and with use of the truncated divisor by div_s64 and div_u64.
I added redundant casts in the div_s64 and div_u64 calls to make it clear what those functions do internally, which should make it easy to understand why we do the same cast in the if statements right above.
I believe this version of the patch addresses everybody's concerns.
---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..6405687 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, unsigned long limit) { long long pos_ratio; + long divisor; long x; + divisor = limit - setpoint; + if (!(s32)divisor) + divisor = 1; /* Avoid div-by-zero */ + x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, - limit - setpoint + 1); + (s32)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 (!(u32)divisor) + divisor = 1; /* Avoid div-by-zero */ + pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty), - x_intercept - bdi_setpoint + 1); + (u32)divisor); } else pos_ratio /= 4;
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