Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom

Andrew Morton
2014-04-30T19:00:06Z

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.

Writing a new suite of div functions sounds overkillish. At some loss
of precision could we do something like

if (divisor > 2^32) {
divisor >>= log2(divisor) - 32;
dividend >>= log2(divisor) - 32;
}
x = div(dividend, divisor);

?

And let's uninline the sorry thing while we're in there ;)