On Wed, 2010-05-26 at 15:40 -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@xxxxxxxxxx>
> Acked-by: Mel Gorman <mel@xxxxxxxxx>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Tested and Acked-by: Larry Woodman <lwoodman@xxxxxxxxxx>
> ---
> 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
> + * 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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