Messages in this thread | | | Date | Wed, 30 Apr 2014 10:26:34 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom |
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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@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 */ >> + > > 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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