Re: [PATCH 3/5] track the root (oldest) anon_vma

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

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