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    SubjectRe: mm, something wring in page_lock_anon_vma_read()?
    On Sat, 20 May 2017, Xishi Qiu wrote:
    > On 2017/5/20 6:00, Hugh Dickins wrote:
    > >
    > > You're ignoring the rcu_read_lock() on entry to page_lock_anon_vma_read(),
    > > and the SLAB_DESTROY_BY_RCU (recently renamed SLAB_TYPESAFE_BY_RCU) nature
    > > of the anon_vma_cachep kmem cache. It is not safe to muck with anon_vma->
    > > root in anon_vma_free(), others could still be looking at it.
    > >
    > > Hugh
    > >
    >
    > Hi Hugh,
    >
    > Thanks for your reply.
    >
    > SLAB_DESTROY_BY_RCU will let it call call_rcu() in free_slab(), but if the
    > anon_vma *reuse* by someone again, access root_anon_vma is not safe, right?

    That is safe, on reuse it is still a struct anon_vma; then the test for
    !page_mapped(page) will show that it's no longer a reliable anon_vma for
    this page, so page_lock_anon_vma_read() returns NULL.

    But of course, if page->_mapcount has been corrupted or misaccounted,
    it may think page_mapped(page) when actually page is not mapped,
    and the anon_vma is not good for it.

    >
    > e.g. if I clean the root pointer before free it, then access root_anon_vma
    > in page_lock_anon_vma_read() is NULL pointer access, right?

    Yes, cleaning root pointer before free may result in NULL pointer access.

    Hugh

    >
    > anon_vma_free()
    > ...
    > anon_vma->root = NULL;
    > kmem_cache_free(anon_vma_cachep, anon_vma);
    > ...
    >
    > Thanks,
    > Xishi Qiu

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