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    SubjectRe: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()
    Date
    > On Fri, 9 Apr 2010 13:16:41 +1000
    > Nick Piggin <npiggin@suse.de> wrote:
    >
    > > On Thu, Apr 08, 2010 at 09:17:39PM +0200, Peter Zijlstra wrote:
    > > > There is nothing preventing the anon_vma from being detached while we
    > > > are spinning to acquire the lock. Most (all?) current users end up
    > > > calling something like vma_address(page, vma) on it, which has a
    > > > fairly good chance of weeding out wonky vmas.
    > > >
    > > > However suppose the anon_vma got freed and re-used while we were
    > > > waiting to acquire the lock, and the new anon_vma fits with the
    > > > page->index (because that is the only thing vma_address() uses to
    > > > determine if the page fits in a particular vma, we could end up
    > > > traversing faulty anon_vma chains.
    > > >
    > > > Close this hole for good by re-validating that page->mapping still
    > > > holds the very same anon_vma pointer after we acquire the lock, if not
    > > > be utterly paranoid and retry the whole operation (which will very
    > > > likely bail, because it's unlikely the page got attached to a different
    > > > anon_vma in the meantime).
    > >
    > > Hm, looks like a bugfix? How was this supposed to be safe?
    > >
    > IIUC.
    >
    > Before Rik's change to anon_vma, once page->mapping is set as anon_vma | 0x1,
    > it's not modified until the page is freed.
    > After the patch, do_wp_page() overwrite page->mapping when it reuse existing
    > page.

    Why?
    IIUC. page->mapping dereference in page_lock_anon_vma() makes four story.

    1. the anon_vma is valid
    -> do page_referenced_one().
    2. the anon_vma is invalid and freed to buddy
    -> bail out by page_mapped(), no touch anon_vma
    3. the anon_vma is kfreed, and not reused
    -> bail out by page_mapped()
    4. the anon_vma is kfreed, but reused as another anon_vma
    -> bail out by page_check_address()

    Now we have to consider 5th story.

    5. the anon_vma is exchanged another anon_vma by do_wp_page.
    -> bail out by above bailing out stuff.


    I agree peter's patch makes sense. but I don't think Rik's patch change
    locking rule.


    >
    > ==
    > static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
    > unsigned long address, pte_t *page_table, pmd_t *pmd,
    > spinlock_t *ptl, pte_t orig_pte)
    > {
    > ....
    > if (PageAnon(old_page) && !PageKsm(old_page)) {
    > if (!trylock_page(old_page)) {
    > page_cache_get(old_page);
    > ....
    > reuse = reuse_swap_page(old_page);
    > if (reuse)
    > /*
    > * The page is all ours. Move it to our anon_vma so
    > * the rmap code will not search our parent or siblings.
    > * Protected against the rmap code by the page lock.
    > */
    > page_move_anon_rmap(old_page, vma, address); ----(*)
    > }
    > ===
    > (*) is new.
    >
    > Then, this new check makes sense in the current kernel.







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