Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

Linus Torvalds
2010-05-05T18:05:24Z

On Wed, 5 May 2010, Mel Gorman wrote:
>
> If the same_vma list is properly ordered then maybe something like the
> following is allowed?

Heh. This is the same logic I just sent out. However:

> + anon_vma = page_rmapping(page);
> + if (!anon_vma)
> + return NULL;
> +
> + spin_lock(&anon_vma->lock);

RCU should guarantee that this spin_lock() is valid, but:

> + /*
> + * Get the oldest anon_vma on the list by depending on the ordering
> + * of the same_vma list setup by __page_set_anon_rmap
> + */
> + avc = list_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);

We're not guaranteed that the 'anon_vma->head' list is non-empty.

Somebody could have freed the list and the anon_vma and we have a stale
'page->anon_vma' (that has just not been _released_ yet).

And shouldn't that be 'list_first_entry'? Or &anon_vma->head.next?

How did that line actually work for you? Or was it just a "it boots", but
no actual testing of the rmap walk?

Linus