On 01/11/2015 02:54 PM, Konstantin Khlebnikov 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")
> 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>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> mm/mmap.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 7b36aa7..12616c5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -778,10 +778,12 @@ again: remove_next = 1 + (end > next->vm_end);
> if (exporter && exporter->anon_vma && !importer->anon_vma) {
> int error;
>
> + importer->anon_vma = exporter->anon_vma;
> error = anon_vma_clone(importer, exporter);
> - if (error)
> + if (error) {
> + importer->anon_vma = NULL;
> return error;
> - importer->anon_vma = exporter->anon_vma;
> + }
> }
> }
>
>
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