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

[Larry Woodman <lwoodman@redhat.com>]

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@redhat.com>
> Acked-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Tested and Acked-by: Larry Woodman <lwoodman@redhat.com>

> ---
>  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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