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Date:	Fri, 25 Jul 2014 13:28:01 +0100
From:	Mel Gorman <>
To:	Vlastimil Babka <>
Cc:, Andrew Morton <>,
	David Rientjes <>,, Minchan Kim <>,
	Joonsoo Kim <>,
	Michal Nazarewicz <>,
	Naoya Horiguchi <>,
	Christoph Lameter <>,
	Rik van Riel <>,
	Zhang Yanfei <>
Subject: Re: [PATCH V4 05/15] mm, compaction: move pageblock checks up from

On Wed, Jul 16, 2014 at 03:48:13PM +0200, Vlastimil Babka wrote:
> isolate_migratepages_range() is the main function of the compaction scanner,
> called either on a single pageblock by isolate_migratepages() during regular
> compaction, or on an arbitrary range by CMA's __alloc_contig_migrate_range().
> It currently perfoms two pageblock-wide compaction suitability checks, and
> because of the CMA callpath, it tracks if it crossed a pageblock boundary in
> order to repeat those checks.
> However, closer inspection shows that those checks are always true for CMA:
> - isolation_suitable() is true because CMA sets cc->ignore_skip_hint to true
> - migrate_async_suitable() check is skipped because CMA uses sync compaction
> We can therefore move the compaction-specific checks to isolate_migratepages()
> and simplify isolate_migratepages_range(). Furthermore, we can mimic the
> freepage scanner family of functions, which has isolate_freepages_block()
> function called both by compaction from isolate_freepages() and by CMA from
> isolate_freepages_range(), where each use-case adds own specific glue code.
> This allows further code simplification.
> Therefore, we rename isolate_migratepages_range() to isolate_freepages_block()
> and limit its functionality to a single pageblock (or its subset). For CMA,
> a new different isolate_migratepages_range() is created as a CMA-specific
> wrapper for the _block() function. The checks specific to compaction are moved
> to isolate_migratepages(). As part of the unification of these two families of
> functions, we remove the redundant zone parameter where applicable, since zone
> pointer is already passed in cc->zone.
> Furthermore, going back to compact_zone() and compact_finished() when pageblock
> is found unsuitable (now by isolate_migratepages()) is wasteful - the checks
> are meant to skip pageblocks quickly. The patch therefore also introduces a
> simple loop into isolate_migratepages() so that it does not return immediately
> on failed pageblock checks, but keeps going until isolate_migratepages_range()
> gets called once. Similarily to isolate_freepages(), the function periodically
> checks if it needs to reschedule or abort async compaction.
> Signed-off-by: Vlastimil Babka <>
> Cc: Minchan Kim <>
> Cc: Mel Gorman <>
> Cc: Joonsoo Kim <>
> Cc: Michal Nazarewicz <>
> Cc: Naoya Horiguchi <>
> Cc: Christoph Lameter <>
> Cc: Rik van Riel <>
> Cc: David Rientjes <>

Acked-by: Mel Gorman <>

Mel Gorman
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