Messages in this thread | | | Date | Wed, 30 Apr 2014 12:35:26 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On Wed, 30 Apr 2014 15:30:04 -0400 Rik van Riel <riel@redhat.com> wrote:
> On 04/30/2014 03:00 PM, Andrew Morton wrote: > > On Wed, 30 Apr 2014 10:41:14 -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 > >> @@ -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); > > > > Doesn't this just paper over the bug one time in four billion? The > > other 3999999999 times, pos_ratio_polynom() returns an incorect result? > > > > If it is indeed the case that pos_ratio_polynom() callers are > > legitimately passing a setpoint which is more than 2^32 less than limit > > then it would be better to handle that input correctly. > > The easy way would be by calling div64_s64 and div64_u64, > which are 64 bit all the way through. > > Any objections?
Sounds good to me.
> The inlined bits seem to be stubs calling the _rem variants > of the functions, and discarding the remainder.
I was referring to pos_ratio_polynom(). The compiler will probably be uninlining it anyway, but still...
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