Re: [PATCH 4/5] always lock the root (oldest) anon_vma

[Mel Gorman <mel@csn.ul.ie>]

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@redhat.com>
> ---
>  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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