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Date:	Fri, 25 Jul 2014 13:23:58 +0100
From:	Mel Gorman <>
To:	Vlastimil Babka <>
Cc:, Andrew Morton <>,
	David Rientjes <>,, Joonsoo Kim <>,
	Michal Nazarewicz <>,
	Naoya Horiguchi <>,
	Christoph Lameter <>,
	Rik van Riel <>,
	Minchan Kim <>,
	Zhang Yanfei <>
Subject: Re: [PATCH V4 04/15] mm, compaction: do not recheck
 suitable_migration_target under lock

On Wed, Jul 16, 2014 at 03:48:12PM +0200, Vlastimil Babka wrote:
> isolate_freepages_block() rechecks if the pageblock is suitable to be a target
> for migration after it has taken the zone->lock. However, the check has been
> optimized to occur only once per pageblock, and compact_checklock_irqsave()
> might be dropping and reacquiring lock, which means somebody else might have
> changed the pageblock's migratetype meanwhile.
> Furthermore, nothing prevents the migratetype to change right after
> isolate_freepages_block() has finished isolating. Given how imperfect this is,
> it's simpler to just rely on the check done in isolate_freepages() without
> lock, and not pretend that the recheck under lock guarantees anything. It is
> just a heuristic after all.
> Signed-off-by: Vlastimil Babka <>
> Reviewed-by: Zhang Yanfei <>
> Acked-by: Minchan Kim <>
> Cc: Mel Gorman <>
> Cc: Joonsoo Kim <>
> Cc: Michal Nazarewicz <>
> Cc: Naoya Horiguchi <>
> Cc: Christoph Lameter <>
> Cc: Rik van Riel <>
> Acked-by: David Rientjes <>

Acked-by: Mel Gorman <>

Mel Gorman
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