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

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, May 26, 2010 at 03:41:24PM -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.
Hi, Rik. 

Hmm, I can understand this point. 
Now, rmap code always depeneds on root anon_vma's lock. 
I think it doesn't depends on KSM and MIGRATION.

If we don't use KSM and MIGRATION and it is compiled out, 
Can root's anon_vma disappear during rmap walking? 
who prevent it?

What am I missing?
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> v2:
>  - merge with -mm and the compaction code
>  - improve the anon_vma refcount comment in anon_vma_fork with the
>    refcount lifetime
> 
>  include/linux/rmap.h |   15 +++++++++++++++
>  mm/ksm.c             |   17 ++++++-----------
>  mm/migrate.c         |   10 +++-------
>  mm/rmap.c            |   45 ++++++++++++++++++++++++++++++++++++++++++++-
>  4 files changed, 68 insertions(+), 19 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
> @@ -81,6 +81,13 @@ static inline int anonvma_external_refco
>  {
>  	return atomic_read(&anon_vma->external_refcount);
>  }
> +
> +static inline void get_anon_vma(struct anon_vma *anon_vma)
> +{
> +	atomic_inc(&anon_vma->external_refcount);
> +}
> +
> +void drop_anon_vma(struct anon_vma *);
>  #else
>  static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
>  {
> @@ -90,6 +97,14 @@ static inline int anonvma_external_refco
>  {
>  	return 0;
>  }
> +
> +static inline void get_anon_vma(struct anon_vma *anon_vma)
> +{
> +}
> +
> +static inline void drop_anon_vma(struct anon_vma *anon_vma)
> +{
> +}
>  #endif /* CONFIG_KSM */
>  
>  static inline struct anon_vma *page_anon_vma(struct page *page)
> Index: linux-2.6.34/mm/ksm.c
> ===================================================================
> --- linux-2.6.34.orig/mm/ksm.c
> +++ linux-2.6.34/mm/ksm.c
> @@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_it
>  			  struct anon_vma *anon_vma)
>  {
>  	rmap_item->anon_vma = anon_vma;
> -	atomic_inc(&anon_vma->external_refcount);
> +	get_anon_vma(anon_vma);
>  }
>  
> -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->external_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 *
>  	 * 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
>  			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(s
>  		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) {
> Index: linux-2.6.34/mm/rmap.c
> ===================================================================
> --- linux-2.6.34.orig/mm/rmap.c
> +++ linux-2.6.34/mm/rmap.c
> @@ -235,6 +235,12 @@ int anon_vma_fork(struct vm_area_struct 
>  	 * lock any of the anon_vmas in this anon_vma tree.
>  	 */
>  	anon_vma->root = pvma->anon_vma->root;
> +	/*
> +	 * With KSM refcounts, an anon_vma can stay around longer than the
> +	 * process it belongs to.  The root anon_vma needs to be pinned
> +	 * until this anon_vma is freed, because the lock lives in the root.
> +	 */
> +	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);
> @@ -264,8 +270,11 @@ static void anon_vma_unlink(struct anon_
>  	empty = list_empty(&anon_vma->head) && !anonvma_external_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)
> @@ -1389,6 +1398,40 @@ int try_to_munlock(struct page *page)
>  		return try_to_unmap_file(page, TTU_MUNLOCK);
>  }
>  
> +#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
> +/*
> + * 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->external_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->external_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():
> Index: linux-2.6.34/mm/migrate.c
> ===================================================================
> --- linux-2.6.34.orig/mm/migrate.c
> +++ linux-2.6.34/mm/migrate.c
> @@ -639,7 +639,7 @@ static int unmap_and_move(new_page_t get
>  			 * exist when the page is remapped later
>  			 */
>  			anon_vma = page_anon_vma(page);
> -			atomic_inc(&anon_vma->external_refcount);
> +			get_anon_vma(anon_vma);
>  		}
>  	}
>  
> @@ -682,12 +682,8 @@ skip_unmap:
>  rcu_unlock:
>  
>  	/* Drop an anon_vma reference if we took one */
> -	if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
> -		int empty = list_empty(&anon_vma->head);
> -		anon_vma_unlock(anon_vma);
> -		if (empty)
> -			anon_vma_free(anon_vma);
> -	}
> +	if (anon_vma)
> +		drop_anon_vma(anon_vma);
>  
>  	if (rcu_locked)
>  		rcu_read_unlock();

-- 
Kind regards,
Minchan Kim