On Wed, May 12, 2010 at 01:40:29PM -0400, Rik van Riel wrote:
> Subject: always lock the root (oldest) anon_vma
>
> Always (and only) lock the root (oldest) anon_vma whenever we do something in an
> anon_vma. The recently introduced anon_vma scalability is due to the rmap code
> scanning only the VMAs that need to be scanned. Many common operations still
> took the anon_vma lock on the root anon_vma, so always taking that lock is not
> expected to introduce any scalability issues.
>
> However, always taking the same lock does mean we only need to take one lock,
> which means rmap_walk on pages from any anon_vma in the vma is excluded from
> occurring during an munmap, expand_stack or other operation that needs to
> exclude rmap_walk and similar functions.
>
> Also add the proper locking to vma_adjust.
>
This last comment is a bit light. It's actually restoring the lock that
was taken in 2.6.33 to some extent except we are always taking it now.
In 2.6.33, it was resricted to
if (vma->anon_vma && (insert || importer || start != vma->vm_start))
anon_vma = vma->anon_vma;
but now it's always. Has it been determined that the locking in 2.6.33
was insufficient or are we playing it safe now?
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> include/linux/rmap.h | 8 ++++----
> mm/ksm.c | 2 +-
> mm/mmap.c | 6 +++++-
> 3 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 457ae1e..33ffe14 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -95,24 +95,24 @@ static inline void vma_lock_anon_vma(struct vm_area_struct *vma)
> {
> struct anon_vma *anon_vma = vma->anon_vma;
> if (anon_vma)
> - spin_lock(&anon_vma->lock);
> + spin_lock(&anon_vma->root->lock);
> }
>
> static inline void vma_unlock_anon_vma(struct vm_area_struct *vma)
> {
> struct anon_vma *anon_vma = vma->anon_vma;
> if (anon_vma)
> - spin_unlock(&anon_vma->lock);
> + spin_unlock(&anon_vma->root->lock);
> }
>
> static inline void anon_vma_lock(struct anon_vma *anon_vma)
> {
> - spin_lock(&anon_vma->lock);
> + spin_lock(&anon_vma->root->lock);
> }
>
> static inline void anon_vma_unlock(struct anon_vma *anon_vma)
> {
> - spin_unlock(&anon_vma->lock);
> + spin_unlock(&anon_vma->root->lock);
> }
>
> /*
> diff --git a/mm/ksm.c b/mm/ksm.c
> index d488012..7ca0dd7 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -325,7 +325,7 @@ static void drop_anon_vma(struct rmap_item *rmap_item)
> {
> struct anon_vma *anon_vma = rmap_item->anon_vma;
>
> - if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->lock)) {
> + if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->root->lock)) {
> int empty = list_empty(&anon_vma->head);
> anon_vma_unlock(anon_vma);
> if (empty)
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f70bc65..b7dfe30 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -553,6 +553,8 @@ again: remove_next = 1 + (end > next->vm_end);
> }
> }
>
> + vma_lock_anon_vma(vma);
> +
> if (file) {
> mapping = file->f_mapping;
> if (!(vma->vm_flags & VM_NONLINEAR))
> @@ -600,6 +602,8 @@ again: remove_next = 1 + (end > next->vm_end);
> flush_dcache_mmap_unlock(mapping);
> }
>
> + vma_unlock_anon_vma(vma);
> +
> if (remove_next) {
> /*
> * vma_merge has merged next into vma, and needs
> @@ -2471,7 +2475,7 @@ static void vm_lock_anon_vma(struct mm_struct *mm, struct anon_vma *anon_vma)
> * The LSB of head.next can't change from under us
> * because we hold the mm_all_locks_mutex.
> */
> - spin_lock_nest_lock(&anon_vma->lock, &mm->mmap_sem);
> + spin_lock_nest_lock(&anon_vma->root->lock, &mm->mmap_sem);
> /*
> * We can safely modify head.next after taking the
> * anon_vma->lock. If some other vma in this mm shares
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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