linux-kernel - Re: [PATCH 02 of 66] mm, migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack

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Date:	Thu, 18 Nov 2010 17:13:39 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 02 of 66] mm, migration: Fix race between
	shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs
	created within the temporary stack

On Thu, Nov 18, 2010 at 11:13:49AM +0000, Mel Gorman wrote:
> > This patch fixes the problem by using two VMAs - one which covers the temporary
> > stack and the other which covers the new location. This guarantees that rmap
> > can always find the migration PTE even if it is copied while rmap_walk is
> > taking place.
> > 
> > Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> 
> This old chestnut. IIRC, this was the more complete solution to a fix that made
> it into mainline. The patch still looks reasonable. It does add a kmalloc()
> but I can't remember if we decided we were ok with it or not. Can you remind
> me? More importantly, it appears to be surviving the original testcase that
> this bug was about (20 minutes so far but will leave it a few hours). Assuming
> the test does not crash;
> 

Incidentally, after 6.5 hours this still hasn't crashed. Previously a
worst case reproduction scenario for the bug was around 35 minutes.

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