Re: [PATCH 03/12] mm: Share the anon_vma ref counts between KSMand page migration

[Mel Gorman]

On Fri, Feb 19, 2010 at 09:18:59AM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 18 Feb 2010 18:02:33 +0000
> Mel Gorman <mel@xxxxxxxxx> wrote:
> 
> > For clarity of review, KSM and page migration have separate refcounts on
> > the anon_vma. While clear, this is a waste of memory. This patch gets
> > KSM and page migration to share their toys in a spirit of harmony.
> > 
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> Nitpick:
> I think this refcnt has something different characteristics than other
> usual refcnts. Even when refcnt goes down to 0, anon_vma will not be freed.
> So, I think some kind of name as temporal_reference_count is better than
> simple "refcnt". Then, it will be clearer what this refcnt is for.
> 

When I read this in a few years, I'll have no idea what "temporal" is
referring to. The holder of this account is by a process that does not
necessarily own the page or its mappings but "remote" has special
meaning as well. "external_count" ?

> 
> > ---
> >  include/linux/rmap.h |   50 ++++++++++++++++++--------------------------------
> >  mm/ksm.c             |    4 ++--
> >  mm/migrate.c         |    4 ++--
> >  mm/rmap.c            |    6 ++----
> >  4 files changed, 24 insertions(+), 40 deletions(-)
> > 
> > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > index 6b5a1a9..55c0e9e 100644
> > --- a/include/linux/rmap.h
> > +++ b/include/linux/rmap.h
> > @@ -26,11 +26,17 @@
> >   */
> >  struct anon_vma {
> >  	spinlock_t lock;	/* Serialize access to vma list */
> > -#ifdef CONFIG_KSM
> > -	atomic_t ksm_refcount;
> > -#endif
> > -#ifdef CONFIG_MIGRATION
> > -	atomic_t migrate_refcount;
> > +#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
> > +
> > +	/*
> > +	 * The refcount is taken by either KSM or page migration
> > +	 * to take a reference to an anon_vma when there is no
> > +	 * guarantee that the vma of page tables will exist for
> > +	 * the duration of the operation. A caller that takes
> > +	 * the reference is responsible for clearing up the
> > +	 * anon_vma if they are the last user on release
> > +	 */
> > +	atomic_t refcount;
> >  #endif
> >  	/*
> >  	 * NOTE: the LSB of the head.next is set by
> > @@ -44,46 +50,26 @@ struct anon_vma {
> >  };
> >  
> >  #ifdef CONFIG_MMU
> > -#ifdef CONFIG_KSM
> > -static inline void ksm_refcount_init(struct anon_vma *anon_vma)
> > +#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
> > +static inline void anonvma_refcount_init(struct anon_vma *anon_vma)
> >  {
> > -	atomic_set(&anon_vma->ksm_refcount, 0);
> > +	atomic_set(&anon_vma->refcount, 0);
> >  }
> >  
> > -static inline int ksm_refcount(struct anon_vma *anon_vma)
> > +static inline int anonvma_refcount(struct anon_vma *anon_vma)
> >  {
> > -	return atomic_read(&anon_vma->ksm_refcount);
> > +	return atomic_read(&anon_vma->refcount);
> >  }
> >  #else
> > -static inline void ksm_refcount_init(struct anon_vma *anon_vma)
> > +static inline void anonvma_refcount_init(struct anon_vma *anon_vma)
> >  {
> >  }
> >  
> > -static inline int ksm_refcount(struct anon_vma *anon_vma)
> > +static inline int anonvma_refcount(struct anon_vma *anon_vma)
> >  {
> >  	return 0;
> >  }
> >  #endif /* CONFIG_KSM */
> > -#ifdef CONFIG_MIGRATION
> > -static inline void migrate_refcount_init(struct anon_vma *anon_vma)
> > -{
> > -	atomic_set(&anon_vma->migrate_refcount, 0);
> > -}
> > -
> > -static inline int migrate_refcount(struct anon_vma *anon_vma)
> > -{
> > -	return atomic_read(&anon_vma->migrate_refcount);
> > -}
> > -#else
> > -static inline void migrate_refcount_init(struct anon_vma *anon_vma)
> > -{
> > -}
> > -
> > -static inline int migrate_refcount(struct anon_vma *anon_vma)
> > -{
> > -	return 0;
> > -}
> > -#endif /* CONFIG_MIGRATE */
> >  
> >  static inline struct anon_vma *page_anon_vma(struct page *page)
> >  {
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 56a0da1..7decf73 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -318,14 +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);
> > +	atomic_inc(&anon_vma->refcount);
> >  }
> >  
> >  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->refcount, &anon_vma->lock)) {
> >  		int empty = list_empty(&anon_vma->head);
> >  		spin_unlock(&anon_vma->lock);
> >  		if (empty)
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 1ce6a2f..00777b0 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -619,7 +619,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> >  		rcu_read_lock();
> >  		rcu_locked = 1;
> >  		anon_vma = page_anon_vma(page);
> > -		atomic_inc(&anon_vma->migrate_refcount);
> > +		atomic_inc(&anon_vma->refcount);
> >  	}
> >  
> >  	/*
> > @@ -661,7 +661,7 @@ skip_unmap:
> >  rcu_unlock:
> >  
> >  	/* Drop an anon_vma reference if we took one */
> > -	if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
> > +	if (anon_vma && atomic_dec_and_lock(&anon_vma->refcount, &anon_vma->lock)) {
> >  		int empty = list_empty(&anon_vma->head);
> >  		spin_unlock(&anon_vma->lock);
> >  		if (empty)
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 11ba74a..96b5905 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -172,8 +172,7 @@ void anon_vma_unlink(struct vm_area_struct *vma)
> >  	list_del(&vma->anon_vma_node);
> >  
> >  	/* We must garbage collect the anon_vma if it's empty */
> > -	empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma) &&
> > -					!migrate_refcount(anon_vma);
> > +	empty = list_empty(&anon_vma->head) && !anonvma_refcount(anon_vma);
> >  	spin_unlock(&anon_vma->lock);
> >  
> >  	if (empty)
> > @@ -185,8 +184,7 @@ static void anon_vma_ctor(void *data)
> >  	struct anon_vma *anon_vma = data;
> >  
> >  	spin_lock_init(&anon_vma->lock);
> > -	ksm_refcount_init(anon_vma);
> > -	migrate_refcount_init(anon_vma);
> > +	anonvma_refcount_init(anon_vma);
> >  	INIT_LIST_HEAD(&anon_vma->head);
> >  }
> >  
> > -- 
> > 1.6.5
> > 
> > --
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> > 
> 

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