Re: [PATCH v4] mm,writeback: fix divide by zero in pos_ratio_polynom

[Rik van Riel <riel@redhat.com>]

On 04/30/2014 04:13 PM, Andrew Morton wrote:
> On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel <riel@redhat.com> wrote:
>
>> I believe this should do the trick.
>>
>> ---8<---
>>
>> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
>>
>> 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.
>
> Changelog is a bit stale.

Will update.

>> -static inline long long pos_ratio_polynom(unsigned long setpoint,
>> +static long long pos_ratio_polynom(unsigned long setpoint,
>>   					  unsigned long dirty,
>>   					  unsigned long limit)
>>   {
>> +	unsigned long divisor;
>>   	long long pos_ratio;
>>   	long x;
>>
>> -	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
>> -		    limit - setpoint + 1);
>> +	divisor = limit - setpoint;
>> +	if (!divisor)
>> +		divisor = 1;	/* Avoid div-by-zero */
>
> This was a consequence of 64->32 truncation and it can't happen any
> more, can it?

That is a good question.  Looking at the code some more,
I guess it may indeed be exclusively due to the truncation,
and we can go back to the older code, just with the fully
64 bit divide functions...

Good thing Masayoshi-san has a reproducer :)

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