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Message-ID: <20100219140500.GG30258@csn.ul.ie>
Date: Fri, 19 Feb 2010 14:05:00 +0000
From: Mel Gorman <mel@....ul.ie>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>,
Christoph Lameter <cl@...ux-foundation.org>,
Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
David Rientjes <rientjes@...gle.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH 03/12] mm: Share the anon_vma ref counts between KSM
and page migration
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@....ul.ie> 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@....ul.ie>
>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> 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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> > Please read the FAQ at http://www.tux.org/lkml/
> >
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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