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    Subject[PATCH 05/28] track the root (oldest) anon_vma
    From: Rik van Riel <riel@redhat.com>

    Track the root (oldest) anon_vma in each anon_vma tree. Because we only
    take the lock on the root anon_vma, we cannot use the lock on higher-up
    anon_vmas to lock anything. This makes it impossible to do an indirect
    lookup of the root anon_vma, since the data structures could go away from
    under us.

    However, a direct pointer is safe because the root anon_vma is always the
    last one that gets freed on munmap or exit, by virtue of the same_vma list
    order and unlink_anon_vmas walking the list forward.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    Acked-by: Mel Gorman <mel@csn.ul.ie>
    Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <20100526154010.3904df5c@annuminas.surriel.com>
    ---
    include/linux/rmap.h | 1 +
    mm/rmap.c | 18 ++++++++++++++++--
    2 files changed, 17 insertions(+), 2 deletions(-)

    Index: linux-2.6/include/linux/rmap.h
    ===================================================================
    --- linux-2.6.orig/include/linux/rmap.h
    +++ linux-2.6/include/linux/rmap.h
    @@ -26,6 +26,7 @@
    */
    struct anon_vma {
    spinlock_t lock; /* Serialize access to vma list */
    + struct anon_vma *root; /* Root of this anon_vma tree */
    #if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)

    /*
    Index: linux-2.6/mm/rmap.c
    ===================================================================
    --- linux-2.6.orig/mm/rmap.c
    +++ linux-2.6/mm/rmap.c
    @@ -132,6 +132,11 @@ int anon_vma_prepare(struct vm_area_stru
    if (unlikely(!anon_vma))
    goto out_enomem_free_avc;
    allocated = anon_vma;
    + /*
    + * This VMA had no anon_vma yet. This anon_vma is
    + * the root of any anon_vma tree that might form.
    + */
    + anon_vma->root = anon_vma;
    }

    anon_vma_lock(anon_vma);
    @@ -224,9 +229,15 @@ int anon_vma_fork(struct vm_area_struct
    avc = anon_vma_chain_alloc();
    if (!avc)
    goto out_error_free_anon_vma;
    - anon_vma_chain_link(vma, avc, anon_vma);
    +
    + /*
    + * The root anon_vm's spinlock is the lock actually used when we
    + * lock any of the anon_vmas in this anon_vma tree.
    + */
    + anon_vma->root = pvma->anon_vma->root;
    /* Mark this anon_vma as the one where our new (COWed) pages go. */
    vma->anon_vma = anon_vma;
    + anon_vma_chain_link(vma, avc, anon_vma);

    return 0;

    @@ -261,7 +272,10 @@ void unlink_anon_vmas(struct vm_area_str
    {
    struct anon_vma_chain *avc, *next;

    - /* Unlink each anon_vma chained to the VMA. */
    + /*
    + * Unlink each anon_vma chained to the VMA. This list is ordered
    + * from newest to oldest, ensuring the root anon_vma gets freed last.
    + */
    list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
    anon_vma_unlink(avc);
    list_del(&avc->same_vma);



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