Messages in this thread Patch in this message | | | Date | Tue, 29 Apr 2014 15:39:36 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On Tue, 29 Apr 2014 15:19:10 -0400 Rik van Riel <riel@redhat.com> wrote:
> 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. > > ... > > --- 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 int divisor;
I'm thinking this would be better as a ulong so I don't have to worry my pretty head over truncation things?
> long long pos_ratio; > long x; > > + divisor = limit - setpoint; > + if (!divisor) > + divisor = 1; > + > 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;
--- a/mm/page-writeback.c~mm-page-writebackc-fix-divide-by-zero-in-pos_ratio_polynom-fix +++ a/mm/page-writeback.c @@ -597,13 +597,13 @@ static inline long long pos_ratio_polyno unsigned long dirty, unsigned long limit) { - unsigned int divisor; + unsigned long divisor; long long pos_ratio; long x; divisor = limit - setpoint; if (!divisor) - divisor = 1; + divisor = 1; /* Avoid div-by-zero */ x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, divisor); _
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