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

[Konstantin Khlebnikov]

On Mon, Jan 12, 2015 at 11:21 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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?

Will do. But let's wait for some just to be sure that is the last bug here.

>
>> 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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