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SubjectRe: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages
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On Mon, 2010-03-15 at 11:28 +0000, Mel Gorman wrote:
> The use after free looks like
> 1. page_mapcount(page) was zero so anon_vma was no longer reliable
> 2. rcu lock taken but the anon_vma at this point can already be garbage because the
> process exited
> 3. call try_to_unmap, looks up tha anon_vma and locks it. This causes problems
>
> I thought the race would be closed but there is still a very tiny window there all
> right. The following alternative should close it. What do you think?
>
> if (PageAnon(page)) {
> rcu_read_lock();
>
> /*
> * If the page has no mappings any more, just bail. An
> * unmapped anon page is likely to be freed soon but worse,
> * it's possible its anon_vma disappeared between when
> * the page was isolated and when we reached here while
> * the RCU lock was not held
> */
> if (!page_mapcount(page)) {
> rcu_read_unlock();
> goto uncharge;
> }
>
> rcu_locked = 1;
> anon_vma = page_anon_vma(page);
> atomic_inc(&anon_vma->external_refcount);
> }
>
> The rcu_unlock label is not used here because the reference counts were not taken in
> the case where page_mapcount == 0.
>

Looks good to me.
Please, repost above code with your use-after-free scenario comment.


--
Kind regards,
Minchan Kim




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