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    Subject[PATCH 03/11] mm: Share the anon_vma ref counts between KSM and page migration
    Date
    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@csn.ul.ie>
    Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
    Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    ---
    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 567d43f..7721674 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 external_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 external_refcount;
    #endif
    /*
    * NOTE: the LSB of the head.next is set by
    @@ -64,46 +70,26 @@ struct anon_vma_chain {
    };

    #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_external_refcount_init(struct anon_vma *anon_vma)
    {
    - atomic_set(&anon_vma->ksm_refcount, 0);
    + atomic_set(&anon_vma->external_refcount, 0);
    }

    -static inline int ksm_refcount(struct anon_vma *anon_vma)
    +static inline int anonvma_external_refcount(struct anon_vma *anon_vma)
    {
    - return atomic_read(&anon_vma->ksm_refcount);
    + return atomic_read(&anon_vma->external_refcount);
    }
    #else
    -static inline void ksm_refcount_init(struct anon_vma *anon_vma)
    +static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
    {
    }

    -static inline int ksm_refcount(struct anon_vma *anon_vma)
    +static inline int anonvma_external_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 a93f1b7..e45ec98 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->external_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->external_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 6eb1efe..e395115 100644
    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -618,7 +618,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,

    rcu_locked = 1;
    anon_vma = page_anon_vma(page);
    - atomic_inc(&anon_vma->migrate_refcount);
    + atomic_inc(&anon_vma->external_refcount);
    }

    /*
    @@ -660,7 +660,7 @@ 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->external_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 578d0fe..af35b75 100644
    --- a/mm/rmap.c
    +++ b/mm/rmap.c
    @@ -248,8 +248,7 @@ static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
    list_del(&anon_vma_chain->same_anon_vma);

    /* 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_external_refcount(anon_vma);
    spin_unlock(&anon_vma->lock);

    if (empty)
    @@ -273,8 +272,7 @@ 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_external_refcount_init(anon_vma);
    INIT_LIST_HEAD(&anon_vma->head);
    }

    --
    1.6.5


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