linux-kernel - Re: [PATCH mmotm 3/3] mm, compaction: prevent nr_isolated

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Date:	Tue, 26 Apr 2016 21:40:45 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michal Hocko <mhocko@...e.com>,
	Hugh Dickins <hughd@...gle.com>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH mmotm 3/3] mm, compaction: prevent nr_isolated_* from
 going negative

On 04/26/2016 02:55 AM, Joonsoo Kim wrote:
> On Mon, Apr 25, 2016 at 03:35:50PM +0200, Vlastimil Babka wrote:
>> @@ -846,9 +845,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>  				spin_unlock_irqrestore(&zone->lru_lock,	flags);
>>  				locked = false;
>>  			}
>> -			putback_movable_pages(migratelist);
>> -			nr_isolated = 0;
>> +			acct_isolated(zone, cc);
>> +			putback_movable_pages(&cc->migratepages);
>> +			cc->nr_migratepages = 0;
>>  			cc->last_migrated_pfn = 0;
>> +			nr_isolated = 0;
>
> Is it better to use separate list and merge it cc->migratepages when
> finishing instead of using cc->migratepages directly? If
> isolate_migratepages() try to isolate more than one page block and keep
> isolated page on previous pageblock, this putback all will invalidate
> all the previous work. It would be beyond of the scope of this
> function. Now, isolate_migratepages() try to isolate the page in one
> pageblock so this code is safe. But, I think that removing such
> dependency will be helpful in the future. I'm not strongly insisting it
> so if you think it's not useful thing, please ignore this comment.

migratelist was merely a reference to cc->migratepages, so it wouldn't prevent 
the situation you are suggesting. A truly separate list would need to be 
appended to cc->migratepages when leaving isolate_migratepages_block() and 
there's no need to do that right now.

BTW, can you check patch 1/3? Thanks!

Vlastimil

> Thanks.
>