Re: [PATCH] mm: fix corner case in anon_vma endless growing prevention

[Andrew Morton]

On Sun, 11 Jan 2015 16:54:06 +0300 Konstantin Khlebnikov <koct9i@xxxxxxxxx> 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@xxxxxxxxx>
> 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@xxxxxxxxxxxxxx>
> Tested-by: Oded Gabbay <oded.gabbay@xxxxxxx>
> Cc: Daniel Forrest <dan.forrest@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
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