Re: [PATCH 03/12] mm: Share the anon_vma ref counts between KSM andpage migration
From: KAMEZAWA Hiroyuki
Date: Thu Feb 18 2010 - 19:26:25 EST
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.
Thanks,
-Kame
> ---
> 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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