Messages in this thread Patch in this message | | | Date | Wed, 30 Apr 2014 12:04:04 +0400 | From | Maxim Patlasov <> | Subject | Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom |
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Hi Rik!
On 04/29/2014 11:19 PM, Rik van Riel 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.
The patch looks correct, but I'm afraid it can hide an actual bug in a caller of pos_ratio_polynom(). The latter is not intended for setpoint > limit. All callers take pains to ensure that setpoint <= limit. Look, for example, at global_dirty_limits():
> if (background >= dirty) > background = dirty / 2;
If you ever encountered "limit - setpoint + 1" equal zero, it may be worthy to investigate how you came to setpoint greater than limit.
Thanks, Maxim
> > Signed-off-by: Rik van Riel <riel@redhat.com> > Cc: stable@vger.kernel.org > --- > mm/page-writeback.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index ef41349..2682516 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 int divisor; > 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;
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