linux-kernel - Re: [PATCH] mm/vmscan.c: fix an implementation flaw in proportional scanning

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Date:	Wed, 18 Jun 2014 11:54:09 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Chen Yucong <slaoub@...il.com>
CC:	akpm@...ux-foundation.org, minchan@...nel.org, mgorman@...e.de,
	hannes@...xchg.org, mhocko@...e.cz, riel@...hat.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan.c: fix an implementation flaw in proportional
 scanning

On 06/18/2014 11:08 AM, Chen Yucong wrote:
> On Wed, 2014-06-18 at 11:00 +0200, Jerome Marchand wrote:
>>>               if (!nr_file || !nr_anon)
>>>                       break;
>>>  
>>> -             if (nr_file > nr_anon) {
>>> -                     unsigned long scan_target =
>> targets[LRU_INACTIVE_ANON] +
>>>
>> -                                             targets[LRU_ACTIVE_ANON]
>> + 1;
>>> +             file_percent = nr_file * 100 / file_target;
>>> +             anon_percent = nr_anon * 100 / anon_target;
>>
>> Here it could happen.
>>
>>
> The snippet 
> 	...
>                if (!nr_file || !nr_anon)
>                       break;

Looks like nr[] values can only decrease and stay positive. Then the
following should be true at all times:

file_target >= nr_file >= 0
anon_target >= nr_anon >= 0

and the code above should indeed avoid the divide by zero.

Thanks,
Jerome

>         ...
>  can help us to filter the situation which you have described. It comes
> from Mel's patch that is called:
> 
> mm: vmscan: use proportional scanning during direct reclaim and full
> scan at DEF_PRIORITY
> 
> thx!
> cyc
> 
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