linux-kernel - Re: [PATCH 2/5] mm: compaction: reset cached scanner pfn's before reading them

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Date:	Tue, 26 Nov 2013 10:23:06 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/5] mm: compaction: reset cached scanner pfn's before
 reading them

On Mon, Nov 25, 2013 at 03:26:07PM +0100, Vlastimil Babka wrote:
> Compaction caches pfn's for its migrate and free scanners to avoid scanning
> the whole zone each time. In compact_zone(), the cached values are read to
> set up initial values for the scanners. There are several situations when
> these cached pfn's are reset to the first and last pfn of the zone,
> respectively. One of these situations is when a compaction has been deferred
> for a zone and is now being restarted during a direct compaction, which is also
> done in compact_zone().
> 
> However, compact_zone() currently reads the cached pfn's *before* resetting
> them. This means the reset doesn't affect the compaction that performs it, and
> with good chance also subsequent compactions, as update_pageblock_skip() is
> likely to be called and update the cached pfn's to those being processed.
> Another chance for a successful reset is when a direct compaction detects that
> migration and free scanners meet (which has its own problems addressed by
> another patch) and sets update_pageblock_skip flag which kswapd uses to do the
> reset because it goes to sleep.
> 
> This is clearly a bug that results in non-deterministic behavior, so this patch
> moves the cached pfn reset to be performed *before* the values are read.
> 
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Rik van Riel <riel@...hat.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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