linux-kernel - Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages

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Date:	Sat, 24 Apr 2010 12:59:37 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of
	PageSwapCache  pages

On Sat, Apr 24, 2010 at 01:13:40PM +0200, Andrea Arcangeli wrote:
> On Sat, Apr 24, 2010 at 11:52:27AM +0100, Mel Gorman wrote:
> > I think you're right. This is a new bug introduced by the anon_vma changes. On
> > the plus side, it means we don't have to worry about -stable.
> 
> Correct, no worry about -stable.
> 
> > > vma_adjust already takes the anon_vma->lock and of course I also
> > > further verified that trying to apply your snippet to vma_adjust
> > > results in immediately deadlock as the very same lock is already taken
> > > in my tree as it's the same anon-vma (simpler).
> > 
> > Yes, I expected that. Previously, there was only one anon_vma so if you
> > double-take the lock, bad things happen.
> > 
> > > So aa.git will be
> > > immune from these bugs for now.
> > > 
> > 
> > It should be. I expect that's why you have never seen the bugon in
> > swapops.
> 
> Correct, I never seen it, and I keep it under very great stress with
> swap storms of hugepages, lots of I/O and khugepaged at 100% cpu.
> 

Well, to me this is also good because it shows it's not an existing bug in
migration or a new bug introduced by compaction either. Previously I hadn't
seen this bug either but until relatively recently, the bulk of the testing
was against 2.6.33.

> Also keep in mind expand_downwards which also adjusts
> vm_start/vm_pgoff the same way (and without mmap_sem write mode).
> 

Will keep it in mind. It's taking the anon_vma lock but once again,
there might be more than one anon_vma to worry about and the proper
locking still isn't massively clear to me.

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