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

[Vlastimil Babka <vbabka@suse.cz>]

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@gmail.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> 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>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  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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