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Date: Mon, 12 Jan 2015 12:21:38 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: linux-mm@kvack.org, Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Rik van Riel <riel@redhat.com>,
        "Elifaz, Dana" <Dana.Elifaz@amd.com>,
        "Bridgman, John" <John.Bridgman@amd.com>,
        Daniel Forrest <dan.forrest@ssec.wisc.edu>,
        Chris Clayton <chris2553@googlemail.com>,
        Oded Gabbay <oded.gabbay@amd.com>, Michal Hocko <mhocko@suse.cz>,
        Greg KH <gregkh@suse.de>
Subject: Re: [PATCH] mm: fix corner case in anon_vma endless growing
 prevention
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On Sun, 11 Jan 2015 16:54:06 +0300 Konstantin Khlebnikov <koct9i@gmail.com> wrote:

> Fix for BUG_ON(anon_vma->degree) splashes in unlink_anon_vmas()
> ("kernel BUG at mm/rmap.c:399!").
> 
> Anon_vma_clone() is usually called for a copy of source vma in destination
> argument. If source vma has anon_vma it should be already in dst->anon_vma.
> NULL in dst->anon_vma is used as a sign that it's called from anon_vma_fork().
> In this case anon_vma_clone() finds anon_vma for reusing.
> 
> Vma_adjust() calls it differently and this breaks anon_vma reusing logic:
> anon_vma_clone() links vma to old anon_vma and updates degree counters but
> vma_adjust() overrides vma->anon_vma right after that. As a result final
> unlink_anon_vmas() decrements degree for wrong anon_vma.
> 
> This patch assigns ->anon_vma before calling anon_vma_clone().
> 
> Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")

I've asked Greg not to take 7a3ef208e662 into -stable because of this
problem.  So if you still think we should fix this in -stable, could
you please prepare an updated patch and send it to Greg?

> Tested-by: Chris Clayton <chris2553@googlemail.com>
> Tested-by: Oded Gabbay <oded.gabbay@amd.com>
> Cc: Daniel Forrest <dan.forrest@ssec.wisc.edu>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Rik van Riel <riel@redhat.com>