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    Subject[PATCH 01/14] mm,migration: Take a reference to the anon_vma before migrating
    Date
    rmap_walk_anon() does not use page_lock_anon_vma() for looking up and
    locking an anon_vma and it does not appear to have sufficient locking to
    ensure the anon_vma does not disappear from under it.

    This patch copies an approach used by KSM to take a reference on the
    anon_vma while pages are being migrated. This should prevent rmap_walk()
    running into nasty surprises later because anon_vma has been freed.

    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    Acked-by: Rik van Riel <riel@redhat.com>
    ---
    include/linux/rmap.h | 23 +++++++++++++++++++++++
    mm/migrate.c | 12 ++++++++++++
    mm/rmap.c | 10 +++++-----
    3 files changed, 40 insertions(+), 5 deletions(-)

    diff --git a/include/linux/rmap.h b/include/linux/rmap.h
    index d25bd22..567d43f 100644
    --- a/include/linux/rmap.h
    +++ b/include/linux/rmap.h
    @@ -29,6 +29,9 @@ struct anon_vma {
    #ifdef CONFIG_KSM
    atomic_t ksm_refcount;
    #endif
    +#ifdef CONFIG_MIGRATION
    + atomic_t migrate_refcount;
    +#endif
    /*
    * NOTE: the LSB of the head.next is set by
    * mm_take_all_locks() _after_ taking the above lock. So the
    @@ -81,6 +84,26 @@ static inline int ksm_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/migrate.c b/mm/migrate.c
    index 6903abf..06e6316 100644
    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -542,6 +542,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
    int rcu_locked = 0;
    int charge = 0;
    struct mem_cgroup *mem = NULL;
    + struct anon_vma *anon_vma = NULL;

    if (!newpage)
    return -ENOMEM;
    @@ -598,6 +599,8 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
    if (PageAnon(page)) {
    rcu_read_lock();
    rcu_locked = 1;
    + anon_vma = page_anon_vma(page);
    + atomic_inc(&anon_vma->migrate_refcount);
    }

    /*
    @@ -637,6 +640,15 @@ skip_unmap:
    if (rc)
    remove_migration_ptes(page, page);
    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)) {
    + int empty = list_empty(&anon_vma->head);
    + spin_unlock(&anon_vma->lock);
    + if (empty)
    + anon_vma_free(anon_vma);
    + }
    +
    if (rcu_locked)
    rcu_read_unlock();
    uncharge:
    diff --git a/mm/rmap.c b/mm/rmap.c
    index fcd593c..578d0fe 100644
    --- a/mm/rmap.c
    +++ b/mm/rmap.c
    @@ -248,7 +248,8 @@ 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);
    + empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma) &&
    + !migrate_refcount(anon_vma);
    spin_unlock(&anon_vma->lock);

    if (empty)
    @@ -273,6 +274,7 @@ static void anon_vma_ctor(void *data)

    spin_lock_init(&anon_vma->lock);
    ksm_refcount_init(anon_vma);
    + migrate_refcount_init(anon_vma);
    INIT_LIST_HEAD(&anon_vma->head);
    }

    @@ -1338,10 +1340,8 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
    /*
    * Note: remove_migration_ptes() cannot use page_lock_anon_vma()
    * because that depends on page_mapped(); but not all its usages
    - * are holding mmap_sem, which also gave the necessary guarantee
    - * (that this anon_vma's slab has not already been destroyed).
    - * This needs to be reviewed later: avoiding page_lock_anon_vma()
    - * is risky, and currently limits the usefulness of rmap_walk().
    + * are holding mmap_sem. Users without mmap_sem are required to
    + * take a reference count to prevent the anon_vma disappearing
    */
    anon_vma = page_anon_vma(page);
    if (!anon_vma)
    --
    1.6.5


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