linux-kernel - Re: [PATCH 1/4] mm: compaction: Ensure that the compaction free scanner does not move to the next zone

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Date:	Wed, 8 Jun 2011 11:33:10 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Thomas Sattler <tsattler@....de>,
	Ury Stankevich <urykhy@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] mm: compaction: Ensure that the compaction free
 scanner does not move to the next zone

On Tue 07-06-11 16:07:02, Mel Gorman wrote:
> Compaction works with two scanners, a migration and a free
> scanner. When the scanners crossover, migration within the zone is
> complete. The location of the scanner is recorded on each cycle to
> avoid excesive scanning.
> 
> When a zone is small and mostly reserved, it's very easy for the
> migration scanner to be close to the end of the zone. Then the following
> situation can occurs
> 
>   o migration scanner isolates some pages near the end of the zone
>   o free scanner starts at the end of the zone but finds that the
>     migration scanner is already there
>   o free scanner gets reinitialised for the next cycle as
>     cc->migrate_pfn + pageblock_nr_pages
>     moving the free scanner into the next zone
>   o migration scanner moves into the next zone
> 
> When this happens, NR_ISOLATED accounting goes haywire because some
> of the accounting happens against the wrong zone. One zones counter
> remains positive while the other goes negative even though the overall
> global count is accurate. This was reported on X86-32 with !SMP because
> !SMP allows the negative counters to be visible. The fact that it is
> difficult to reproduce on X86-64 is probably just a co-incidence as
> the bug should theoritically be possible there.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> Reviewed-by: Minchan Kim <minchan.kim@...il.com>

Reviewed-by: Michal Hocko <mhocko@...e.cz>

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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