linux-kernel - Re: [patch v3 2/6] mm, compaction: return failed migration target pages back to freelist

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Message-Id: <20140507141534.d4def933b3a9999e7826df5c@linux-foundation.org>
Date:	Wed, 7 May 2014 14:15:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch v3 2/6] mm, compaction: return failed migration target
 pages back to freelist

On Tue, 6 May 2014 19:22:43 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> Memory compaction works by having a "freeing scanner" scan from one end of a 
> zone which isolates pages as migration targets while another "migrating scanner" 
> scans from the other end of the same zone which isolates pages for migration.
> 
> When page migration fails for an isolated page, the target page is returned to 
> the system rather than the freelist built by the freeing scanner.  This may 
> require the freeing scanner to continue scanning memory after suitable migration 
> targets have already been returned to the system needlessly.
> 
> This patch returns destination pages to the freeing scanner freelist when page 
> migration fails.  This prevents unnecessary work done by the freeing scanner but 
> also encourages memory to be as compacted as possible at the end of the zone.
> 
> Reported-by: Greg Thelen <gthelen@...gle.com>

What did Greg actually report?  IOW, what if any observable problem is
being fixed here?
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