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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Message-ID: <20101119173817.GE24450@random.random>
Date:	Fri, 19 Nov 2010 18:38:17 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
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 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

We decided the kmalloc was ok, but Linus didn't like this approach. I
kept it in my tree because I didn't want to remember when/if to add the
special check in the accurate rmap walks. I find it simpler if all
rmap walks are accurate by default.

> 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;

Sure the patch is safe.

If Linus still doesn't like this, I will immediately remove this patch
and add the special checks to the rmap walks in huge_memory.c, you
know my preference but this is a detail and my preference is
irrelevant.

Thanks,
Andrea
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