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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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