Re: [PATCH 5/5] extend KSM refcounts to the anon_vma root

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

On Wed, May 12, 2010 at 01:41:11PM -0400, Rik van Riel wrote:
> Subject: extend KSM refcounts to the anon_vma root
> 
> KSM reference counts can cause an anon_vma to exist after the processe
> it belongs to have already exited.  Because the anon_vma lock now lives
> in the root anon_vma, we need to ensure that the root anon_vma stays
> around until after all the "child" anon_vmas have been freed.
> 
> The obvious way to do this is to have a "child" anon_vma take a
> reference to the root in anon_vma_fork.  When the anon_vma is freed
> at munmap or process exit, we drop the refcount in anon_vma_unlink
> and possibly free the root anon_vma.
> 
> The KSM anon_vma reference count function also needs to be modified
> to deal with the possibility of freeing 2 levels of anon_vma.  The
> easiest way to do this is to break out the KSM magic and make it
> generic.
> 
> When compiling without CONFIG_KSM, this code is compiled out.
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
>  include/linux/rmap.h |   12 ++++++++++++
>  mm/ksm.c             |   17 ++++++-----------
>  mm/rmap.c            |   45 ++++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 62 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 33ffe14..387d40c 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -126,6 +126,18 @@ int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
>  void __anon_vma_link(struct vm_area_struct *);
>  void anon_vma_free(struct anon_vma *);
>  
> +#ifdef CONFIG_KSM
> +static inline void get_anon_vma(struct anon_vma *anon_vma)
> +{
> +	atomic_inc(&anon_vma->ksm_refcount);
> +}
> +
> +void drop_anon_vma(struct anon_vma *);
> +#else
> +#define get_anon_vma(x)		do {} while(0)
> +#define drop_anon_vma(x)	do {} while(0)
> +#endif
> +
>  static inline void anon_vma_merge(struct vm_area_struct *vma,
>  				  struct vm_area_struct *next)
>  {
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 7ca0dd7..9f2acc9 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_item *rmap_item,
>  			  struct anon_vma *anon_vma)
>  {
>  	rmap_item->anon_vma = anon_vma;
> -	atomic_inc(&anon_vma->ksm_refcount);
> +	get_anon_vma(anon_vma);
>  }

I'm not quite getting this. Here, we get the local anon_vma so we
increment its reference count and later we drop it but without a
refcount taken on the root anon_vma, why is it guaranteed to stay
around?

>  
> -static void drop_anon_vma(struct rmap_item *rmap_item)
> +static void ksm_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->root->lock)) {
> -		int empty = list_empty(&anon_vma->head);
> -		anon_vma_unlock(anon_vma);
> -		if (empty)
> -			anon_vma_free(anon_vma);
> -	}
> +	drop_anon_vma(anon_vma);
>  }
>  
>  /*
> @@ -415,7 +410,7 @@ static void break_cow(struct rmap_item *rmap_item)
>  	 * It is not an accident that whenever we want to break COW
>  	 * to undo, we also need to drop a reference to the anon_vma.
>  	 */
> -	drop_anon_vma(rmap_item);
> +	ksm_drop_anon_vma(rmap_item);
>  
>  	down_read(&mm->mmap_sem);
>  	if (ksm_test_exit(mm))
> @@ -470,7 +465,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
>  			ksm_pages_sharing--;
>  		else
>  			ksm_pages_shared--;
> -		drop_anon_vma(rmap_item);
> +		ksm_drop_anon_vma(rmap_item);
>  		rmap_item->address &= PAGE_MASK;
>  		cond_resched();
>  	}
> @@ -558,7 +553,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
>  		else
>  			ksm_pages_shared--;
>  
> -		drop_anon_vma(rmap_item);
> +		ksm_drop_anon_vma(rmap_item);
>  		rmap_item->address &= PAGE_MASK;
>  
>  	} else if (rmap_item->address & UNSTABLE_FLAG) {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f0ba648..d63cd91 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -238,6 +238,12 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>  	 */
>  	root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
>  	anon_vma->root = root_avc->anon_vma;
> +	/*
> +	 * With KSM refcounts, an anon_vma can stay around longer than the
> +	 * process it belongs to.  The root anon_vma needs to be pinned,
> +	 * because that is where the lock lives.
> +	 */
> +	get_anon_vma(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);
> @@ -267,8 +273,11 @@ static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
>  	empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma);
>  	anon_vma_unlock(anon_vma);
>  
> -	if (empty)
> +	if (empty) {
> +		/* We no longer need the root anon_vma */
> +		drop_anon_vma(anon_vma->root);
>  		anon_vma_free(anon_vma);
> +	}
>  }
>  
>  void unlink_anon_vmas(struct vm_area_struct *vma)
> @@ -1355,6 +1364,40 @@ int try_to_munlock(struct page *page)
>  		return try_to_unmap_file(page, TTU_MUNLOCK);
>  }
>  
> +#ifdef CONFIG_KSM
> +/*
> + * Drop an anon_vma refcount, freeing the anon_vma and anon_vma->root
> + * if necessary.  Be careful to do all the tests under the lock.  Once
> + * we know we are the last user, nobody else can get a reference and we
> + * can do the freeing without the lock.
> + */
> +void drop_anon_vma(struct anon_vma *anon_vma)
> +{
> +	if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->root->lock)) {
> +		struct anon_vma *root = anon_vma->root;
> +		int empty list_empty(&anon_vma->head);
> +		int last_root_user = 0;
> +		int root_empty = 0;
> +
> +		/*
> +		 * The refcount on a non-root anon_vma got dropped.  Drop
> +		 * the refcount on the root and check if we need to free it.
> +		 */
> +		if (empty && anon_vma != root) {
> +			last_root_user = atomic_dec_and_test(&root->ksm_refcount);
> +			root_empty = list_empty(&root->head);
> +		}
> +		anon_vma_unlock(anon_vma);
> +
> +		if (empty) {
> +			anon_vma_free(anon_vma);
> +			if (root_empty && last_root_user)
> +				anon_vma_free(root);
> +		}
> +	}
> +}
> +#endif
> +
>  #ifdef CONFIG_MIGRATION
>  /*
>   * rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
> 

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