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

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Date:	Mon, 3 Feb 2014 18:50:54 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....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 Tue, 4 Feb 2014, Joonsoo Kim wrote:

> > > 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.
> > > 
> > 
> > So how does that work with migrate_page_move_mapping() which demands 
> > page_count(page) == 1 and the get_page_unless_zero() in 
> > __isolate_lru_page()?
> 
> Before doing migrate_page_move_mapping(), try_to_unmap() is called so that all
> mapping is unmapped. Then, remained page_count() is 1 which is grabbed by
> __isolate_lru_page(). Am I missing something?
> 

Ah, good point.  I wonder if we can get away with 
page_count(page) - page_mapcount(page) > 1 to avoid the get_user_pages() 
pin?
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