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

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Date:	Wed, 5 May 2010 08:31:42 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	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>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On Wed, 5 May 2010, Mel Gorman wrote:
> 
> rmap_walk() appears to be the only one that takes multiple locks but it itself
> is not serialised. If there are more than one process calling rmap_walk()
> on different processes sharing the same VMAs, is there a guarantee they walk
> it in the same order?

So I had this notion of the list always getting deeper and us guaranteeing 
the order in it, but you're right - that's not the 'same_anon_vma' list, 
it's the 'same_vma' one.

Damn. So yeah, I don't see us guaranteeing any ordering guarantees. My 
bad.

That said, I do wonder if we could _make_ the ordering reliable. I did 
that for the 'same_vma' one, because I wanted to be able to verify that 
chains were consistent (and we also needed to be able to find the "oldest 
anon_vma" for the case of re-instantiating pages that migth exist in 
multiple different anon_vma's).

Any ideas?

		Linus
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