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 07:34:37 -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:
>
> With the recent anon_vma changes, there can be more than one anon_vma->lock
> to take in a anon_vma_chain but a second lock cannot be spinned upon in case
> of deadlock. The rmap walker tries to take locks of different anon_vma's
> but if the attempt fails, locks are released and the operation is restarted.

Btw, is this really needed?

Nobody else takes two anon_vma locks at the same time, so in order to 
avoid ABBA deadlocks all we need to guarantee is that rmap_walk_ksm() and 
rmap_walk_anon() always lock the anon_vma's in the same order.

And they do, as far as I can tell. How could we ever get a deadlock when 
we have both cases doing the locking by walking the same_anon_vma list?

	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {

So I think the "retry" logic looks unnecessary, and actually opens us up 
to a possible livelock bug (imagine a long chain, and heavy page fault 
activity elsewhere that ends up locking some anon_vma in the chain, and 
just the right behavior that gets us into a lockstep situation), rather 
than fixing an ABBA deadlock.

Now, if it's true that somebody else _does_ do nested anon_vma locking, 
I'm obviously wrong. But I don't see such usage.

Comments?

				Linus
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