linux-kernel - Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Message-ID: <20100319085949.GQ12388@csn.ul.ie>
Date:	Fri, 19 Mar 2010 08:59:49 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped
	anonymous pages

On Fri, Mar 19, 2010 at 03:21:41PM +0900, KOSAKI Motohiro wrote:
> > > then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount.
> > > So, I think we don't need your [1/11] patch.
> > > 
> > > Am I missing something?
> > > 
> > 
> > The refcount is still needed. The anon_vma might be valid, but the
> > refcount is what ensures that the anon_vma is not freed and reused.
> 
> please please why do we need both mechanism. now cristoph is very busy and I am
> de fact reviewer of page migration and mempolicy code. I really hope to understand
> your patch.
> 

As in, why not drop the RCU protection of anon_vma altogeter? Mainly, because I
think it would be reaching too far for this patchset and it should be done as
a follow-up. Putting the ref-count everywhere will change the cache-behaviour
of anon_vma more than I'd like to slip into a patchset like this. Secondly,
Christoph mentions that SLAB_DESTROY_BY_RCU is used to keep anon_vma cache-hot.
For these reasons, removing RCU from these paths and adding the refcount
in others is a patch that should stand on its own.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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