linux-kernel - Re: [patch] mm, compaction: avoid isolating pinned pages fix

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Date:	Tue, 4 Feb 2014 10:53:32 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch] mm, compaction: avoid isolating pinned pages fix

On Mon, Feb 03, 2014 at 05:20:46PM -0800, David Rientjes wrote:
> On Tue, 4 Feb 2014, Joonsoo Kim wrote:
> 
> > I think that you need more code to skip this type of page correctly.
> > Without page_mapped() check, this code makes migratable pages be skipped,
> > since if page_mapped() case, page_count() may be more than zero.
> > 
> > So I think that you need following change.
> > 
> > (!page_mapping(page) && !page_mapped(page) && page_count(page))
> > 
> 
> These pages returned by get_user_pages() will have a mapcount of 1 so this 
> wouldn't actually fix the massive lock contention.  page_mapping() is only 
> going to be NULL for pages off the lru like these are for 
> PAGE_MAPPING_ANON.

Okay. It can't fix your situation. Anyway, *normal* anon pages may be mapped
and have positive page_count(), so your code such as
'!page_mapping(page) && page_count(page)' makes compaction skip these *normal*
anon pages and this is incorrect behaviour.

Thanks.
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