linux-kernel - Re: [PATCH 2/2] 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: <20100507105712.18fc90c4.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 7 May 2010 10:57:12 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and
 rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary
 stack

On Thu, 6 May 2010 18:40:39 -0700 (PDT)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Fri, 7 May 2010, Mel Gorman wrote:
> > 
> > Page migration requires rmap to be able to find all migration ptes
> > created by migration. If the second rmap_walk clearing migration PTEs
> > misses an entry, it is left dangling causing a BUG_ON to trigger during
> > fault. For example;
> 
> So I still absolutely detest this patch.
> 
> Why didn't the other - much simpler - patch work? The one Rik pointed to:
> 
> 	http://lkml.org/lkml/2010/4/30/198
> 
> and didn't do that _disgusting_ temporary anon_vma?
> 
I vote for simple one rather than temporal anon_vma. IIUC, this patch is selected
for not to leak exec's problem out to mm/ by magical check.


> Alternatively, why don't we just take the anon_vma lock over this region, 
> so that rmap can't _walk_ the damn thing?
> 

IIUC, move_page_tables() may call "page table allocation" and it cannot be
done under spinlock.

Thanks,
-Kame

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