Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom

Andrew Morton
2014-04-29T22:53:56Z

On Tue, 29 Apr 2014 18:48:11 -0400 Rik van Riel <riel@redhat.com> wrote:

> On 04/29/2014 06:39 PM, Andrew Morton wrote:
> > 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?
>
> I looked at div_*64, and the second argument is a 32 bit
> variable. I guess a long would be ok, since if we are
> dividing by more than 4 billion we don't really care :)
>
> static inline s64 div_s64(s64 dividend, s32 divisor)

ah, good point. Switching to ulong is perhaps a bit misleading then.