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SubjectRe: [PATCH 3/5] track the root (oldest) anon_vma
On Wed, May 12, 2010 at 01:39:58PM -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.
>

Shouldn't this be "usually the last one that gets freed" because of the
ref-counting by KSM aspect? Minor nit anyway.

> Signed-off-by: Rik van Riel <riel@redhat.com>

Otherwise

Acked-by: Mel Gorman <mel@csn.ul.ie>

> ---
> include/linux/rmap.h | 1 +
> mm/rmap.c | 20 +++++++++++++++++---
> 2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 72ecd87..457ae1e 100644
> --- a/include/linux/rmap.h
> +++ b/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 */
> #ifdef CONFIG_KSM
> atomic_t ksm_refcount;
> #endif
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 6102f77..e34cb56 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -132,6 +132,11 @@ int anon_vma_prepare(struct vm_area_struct *vma)
> 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);
> @@ -203,7 +208,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> */
> int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> {
> - struct anon_vma_chain *avc;
> + struct anon_vma_chain *avc, *root_avc;
> struct anon_vma *anon_vma;
>
> /* Don't bother if the parent process has no anon_vma here. */
> @@ -224,9 +229,18 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> avc = anon_vma_chain_alloc();
> if (!avc)
> goto out_error_free_anon_vma;
> - anon_vma_chain_link(vma, avc, anon_vma);
> +
> + /*
> + * Get the root anon_vma on the list by depending on the ordering
> + * of the same_vma list setup by previous invocations of anon_vma_fork.
> + * The root anon_vma will always be referenced by the last item
> + * in the anon_vma_chain list.
> + */
> + root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> + anon_vma->root = root_avc->anon_vma;
> /* 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 +275,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> {
> struct anon_vma_chain *avc, *next;
>
> - /* Unlink each anon_vma chained to the VMA. */
> + /* Unlink each anon_vma chained to the VMA, from newest to oldest. */
> list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
> anon_vma_unlink(avc);
> list_del(&avc->same_vma);
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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