linux-kernel - Re: [patch] mm, thp: abort compaction if migration page cannot be charged to memcg

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Date:	Mon, 25 Jun 2012 16:40:45 -0400
From:	Rik van Riel <riel@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch] mm, thp: abort compaction if migration page cannot be
 charged to memcg

On 06/21/2012 02:52 AM, David Rientjes wrote:
> If page migration cannot charge the new page to the memcg,
> migrate_pages() will return -ENOMEM.  This isn't considered in memory
> compaction however, and the loop continues to iterate over all pageblocks
> trying in a futile attempt to continue migrations which are only bound to
> fail.
>
> This will short circuit and fail memory compaction if migrate_pages()
> returns -ENOMEM.  COMPACT_PARTIAL is returned in case some migrations
> were successful so that the page allocator will retry.

The patch makes sense, however I wonder if it would make
more sense in the long run to allow migrate/compaction to
temporarily exceed the memcg memory limit for a cgroup,
because the original page will get freed again soon anyway.

That has the potential to improve compaction success, and
reduce compaction related CPU use.
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