linux-kernel - Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma

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Message-ID: <20100503171837.GG19891@random.random>
Date:	Mon, 3 May 2010 19:18:37 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Rik van Riel <riel@...hat.com>, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, Mel Gorman <mel@....ul.ie>,
	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>
Subject: Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma_lock

On Mon, May 03, 2010 at 07:11:19PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-05-03 at 19:02 +0200, Andrea Arcangeli wrote:
> > On Mon, May 03, 2010 at 06:55:12PM +0200, Peter Zijlstra wrote:
> > > This does leave me worrying about concurrent faults poking at
> > > vma->vm_end without synchronization.
> > 
> > I didn't check this patch in detail yet. I agree it can be removed and
> > I think it can be safely replaced with the page_table_lock.
> 
> Sure, it could probably be replaced with the ptl, but a single
> anon_vma->lock would I think be better since there's more of them.

ptl not enough, or it'd break if stack grows fast more than the size
of one pmd, page_table_lock enough instead.

Keeping anon_vma lock is sure fine with me ;), I was informally asked
if it was a must have, and I couldn't foresee any problem in
_replacing_ it (not removing) with page_table_lock (which I hope I
mentioned in my answer ;). But I never had an interest to remove it,
just I couldn't find any good reason to keep it either other than
"paranoid just in case", which is good enough justification to me ;)
considering these archs are uncommon and by definition gets less
testing.
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