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    SubjectRe: [PATCH] mm: fix hang on anon_vma->root->lock
    On Thu, Aug 26, 2010 at 03:54:28AM -0700, Hugh Dickins wrote:
    > On Wed, Aug 25, 2010 at 11:41 PM, David Miller <> wrote:
    > > From: Hugh Dickins <>
    > > Date: Wed, 25 Aug 2010 23:12:54 -0700 (PDT)
    > >
    > >> After several hours, kbuild tests hang with anon_vma_prepare() spinning on
    > >> a newly allocated anon_vma's lock - on a box with CONFIG_TREE_PREEMPT_RCU=y
    > >> (which makes this very much more likely, but it could happen without).
    > >>
    > >> The ever-subtle page_lock_anon_vma() now needs a further twist: since
    > >> anon_vma_prepare() and anon_vma_fork() are liable to change the ->root
    > >> of a reused anon_vma structure at any moment, page_lock_anon_vma()
    > >> needs to check page_mapped() again before succeeding, otherwise
    > >> page_unlock_anon_vma() might address a different root->lock.
    > >>
    > >> Signed-off-by: Hugh Dickins <>
    > >
    > > Interesting, is the condition which allows this to trigger specific
    > > to this merge window or was it always possible?
    > Just specific to this merge window, which started using
    > anon_vma->root->lock in place of anon_vma->lock (anon_vma->root is
    > often anon_vma itself, but not always). I _think_ that change was
    > itself a simplification of the locking in 2.6.35, rather than plugging
    > a particular hole (it's not been backported to -stable), but I may be
    > wrong on that - Rik?

    rmap_walk_anon isn't stable without the anon_vma->root->lock. This is
    because it only locks the local anon_vma and but the "vma" can be
    still modified under it in vma_adjust, so vma_address can fail and
    migration will crash. swapping is not issue as it's ok to miss a pte
    once in a while if the vma is under vma_adjust by the time the VM
    tries to unmap the page.

    See the anon_vma_lock added as well to vma_adjust (without it, it'd be
    just an useless exercise, but instead it's really plugging a real hole
    thanks to the anon_vma_lock addition to vma_adjust).

    Again not an issue unless you use migration (incidentally what memory
    compaction uses, and you know who the primary user of memory
    compaction is :).

    Secondly ksm_does_need_to_copy can't be fixed to cow only pages that
    are nonlinear as the page->mapping != anon_vma can now generate false
    positives. It's now trivial to fix with page->mapping->root !=

    About your patch, it's a noop in my view... A single page_mapping
    check after rcu_read_lock is enough. And "anon_vma->root" can't change
    if page->mapping points to "anon_vma" at any time after rcu_read_lock
    returns. rcu_read_lock works for all anon_vma including

    If you were running the page_mapped() check on a different "page" then
    it could make a difference, but repeating it on the same page in the
    same rcu_read_lock protected critical section won't make a difference
    as far as the anon_vma freeing is concerned.

     \ /
      Last update: 2010-08-27 02:23    [W:0.027 / U:0.064 seconds]
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