linux-kernel - Re: [PATCH v4] mm,writeback: fix divide by zero in pos_ratio

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Message-ID: <53615DEE.90808@redhat.com>
Date:	Wed, 30 Apr 2014 16:32:46 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Michal Hocko <mhocko@...e.cz>,
	Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	mpatlasov@...allels.com, Motohiro.Kosaki@...fujitsu.com
Subject: Re: [PATCH v4] mm,writeback: fix divide by zero in pos_ratio_polynom

On 04/30/2014 04:13 PM, Andrew Morton wrote:
> On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel <riel@...hat.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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