linux-kernel - Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

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Date:	Thu, 06 May 2010 09:40:56 -0400
From:	Rik van Riel <riel@...hat.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>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On 05/05/2010 01:34 PM, Linus Torvalds wrote:

>   - you always lock the _deepest_ anon_vma you can find.

The emphasis should be on "always" :)

> That means just a single lock. And the "deepest" anon_vma is well-defined
> for all anon_vma's, because each same_anon_vma chain is always rooted in
> the original anon_vma that caused it.

It should work, but only if we always take the deepest
anon_vma lock.

Not just in the migration code, but also in mmap, munmap,
mprotect (for split_vma), expand_stack, etc...

Otherwise we will still not provide exclusion of migrate
vs. those events.

I'm guessing that means changing both anon_vma_lock and
page_lock_anon_vma to always take the deepest anon_vma
lock - not introducing a new function that is only called
by the migration code.

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