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Date: Wed, 28 Apr 2010 17:39:12 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Mel Gorman <mel@csn.ul.ie>
Cc: Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
       Minchan Kim <minchan.kim@gmail.com>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 2/3] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information
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Another way (not sure if it's good or bad, but it'd clearly avoid the
restarting locks in rmap_walk) would be to allocate a shared lock, and
still share the lock like the anon_vma was shared before. So we have
shorter chains to walk, but still a larger lock, so we've to take just
one and be safe.

spin_lock(&vma->anon_vma->shared_anon_vma_lock->lock)

something like that...