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

[Andrew Morton <akpm@linux-foundation.org>]

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 ;)

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