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    SubjectRe: [PATCH 3/5] track the root (oldest) anon_vma
    On Wed, May 26, 2010 at 03:40:10PM -0400, Rik van Riel wrote:
    > Subject: track the root (oldest) anon_vma
    >
    > 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>
    Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

    Except below one minor type.

    > ---
    > include/linux/rmap.h | 1 +
    > mm/rmap.c | 18 ++++++++++++++++--
    > 2 files changed, 17 insertions(+), 2 deletions(-)
    >
    > Index: linux-2.6.34/include/linux/rmap.h
    > ===================================================================
    > --- linux-2.6.34.orig/include/linux/rmap.h
    > +++ linux-2.6.34/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-2.6.34/mm/rmap.c
    > ===================================================================
    > --- linux-2.6.34.orig/mm/rmap.c
    > +++ linux-2.6.34/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
    anon_vma's
    --
    Kind regards,
    Minchan Kim


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