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    Subject[PATCH 07/28] extend KSM refcounts to the anon_vma root
    From: Rik van Riel <riel@redhat.com>

    KSM reference counts can cause an anon_vma to exist after the processe
    it belongs to have already exited. Because the anon_vma lock now lives
    in the root anon_vma, we need to ensure that the root anon_vma stays
    around until after all the "child" anon_vmas have been freed.

    The obvious way to do this is to have a "child" anon_vma take a
    reference to the root in anon_vma_fork. When the anon_vma is freed
    at munmap or process exit, we drop the refcount in anon_vma_unlink
    and possibly free the root anon_vma.

    The KSM anon_vma reference count function also needs to be modified
    to deal with the possibility of freeing 2 levels of anon_vma. The
    easiest way to do this is to break out the KSM magic and make it
    generic.

    When compiling without CONFIG_KSM, this code is compiled out.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <20100526154124.04607d04@annuminas.surriel.com>
    ---
    v2:
    - merge with -mm and the compaction code
    - improve the anon_vma refcount comment in anon_vma_fork with the
    refcount lifetime

    include/linux/rmap.h | 15 +++++++++++++++
    mm/ksm.c | 17 ++++++-----------
    mm/migrate.c | 10 +++-------
    mm/rmap.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
    4 files changed, 68 insertions(+), 19 deletions(-)

    Index: linux-2.6/include/linux/rmap.h
    ===================================================================
    --- linux-2.6.orig/include/linux/rmap.h
    +++ linux-2.6/include/linux/rmap.h
    @@ -81,6 +81,13 @@ static inline int anonvma_external_refco
    {
    return atomic_read(&anon_vma->external_refcount);
    }
    +
    +static inline void get_anon_vma(struct anon_vma *anon_vma)
    +{
    + atomic_inc(&anon_vma->external_refcount);
    +}
    +
    +void drop_anon_vma(struct anon_vma *);
    #else
    static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
    {
    @@ -90,6 +97,14 @@ static inline int anonvma_external_refco
    {
    return 0;
    }
    +
    +static inline void get_anon_vma(struct anon_vma *anon_vma)
    +{
    +}
    +
    +static inline void drop_anon_vma(struct anon_vma *anon_vma)
    +{
    +}
    #endif /* CONFIG_KSM */

    static inline struct anon_vma *page_anon_vma(struct page *page)
    Index: linux-2.6/mm/ksm.c
    ===================================================================
    --- linux-2.6.orig/mm/ksm.c
    +++ linux-2.6/mm/ksm.c
    @@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_it
    struct anon_vma *anon_vma)
    {
    rmap_item->anon_vma = anon_vma;
    - atomic_inc(&anon_vma->external_refcount);
    + get_anon_vma(anon_vma);
    }

    -static void drop_anon_vma(struct rmap_item *rmap_item)
    +static void ksm_drop_anon_vma(struct rmap_item *rmap_item)
    {
    struct anon_vma *anon_vma = rmap_item->anon_vma;

    - if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
    - int empty = list_empty(&anon_vma->head);
    - anon_vma_unlock(anon_vma);
    - if (empty)
    - anon_vma_free(anon_vma);
    - }
    + drop_anon_vma(anon_vma);
    }

    /*
    @@ -415,7 +410,7 @@ static void break_cow(struct rmap_item *
    * It is not an accident that whenever we want to break COW
    * to undo, we also need to drop a reference to the anon_vma.
    */
    - drop_anon_vma(rmap_item);
    + ksm_drop_anon_vma(rmap_item);

    down_read(&mm->mmap_sem);
    if (ksm_test_exit(mm))
    @@ -470,7 +465,7 @@ static void remove_node_from_stable_tree
    ksm_pages_sharing--;
    else
    ksm_pages_shared--;
    - drop_anon_vma(rmap_item);
    + ksm_drop_anon_vma(rmap_item);
    rmap_item->address &= PAGE_MASK;
    cond_resched();
    }
    @@ -558,7 +553,7 @@ static void remove_rmap_item_from_tree(s
    else
    ksm_pages_shared--;

    - drop_anon_vma(rmap_item);
    + ksm_drop_anon_vma(rmap_item);
    rmap_item->address &= PAGE_MASK;

    } else if (rmap_item->address & UNSTABLE_FLAG) {
    Index: linux-2.6/mm/rmap.c
    ===================================================================
    --- linux-2.6.orig/mm/rmap.c
    +++ linux-2.6/mm/rmap.c
    @@ -235,6 +235,12 @@ int anon_vma_fork(struct vm_area_struct
    * lock any of the anon_vmas in this anon_vma tree.
    */
    anon_vma->root = pvma->anon_vma->root;
    + /*
    + * With KSM refcounts, an anon_vma can stay around longer than the
    + * process it belongs to. The root anon_vma needs to be pinned
    + * until this anon_vma is freed, because the lock lives in the root.
    + */
    + get_anon_vma(anon_vma->root);
    /* Mark this anon_vma as the one where our new (COWed) pages go. */
    vma->anon_vma = anon_vma;
    anon_vma_chain_link(vma, avc, anon_vma);
    @@ -264,8 +270,11 @@ static void anon_vma_unlink(struct anon_
    empty = list_empty(&anon_vma->head) && !anonvma_external_refcount(anon_vma);
    anon_vma_unlock(anon_vma);

    - if (empty)
    + if (empty) {
    + /* We no longer need the root anon_vma */
    + drop_anon_vma(anon_vma->root);
    anon_vma_free(anon_vma);
    + }
    }

    void unlink_anon_vmas(struct vm_area_struct *vma)
    @@ -1382,6 +1391,40 @@ int try_to_munlock(struct page *page)
    return try_to_unmap_file(page, TTU_MUNLOCK);
    }

    +#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
    +/*
    + * Drop an anon_vma refcount, freeing the anon_vma and anon_vma->root
    + * if necessary. Be careful to do all the tests under the lock. Once
    + * we know we are the last user, nobody else can get a reference and we
    + * can do the freeing without the lock.
    + */
    +void drop_anon_vma(struct anon_vma *anon_vma)
    +{
    + if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
    + struct anon_vma *root = anon_vma->root;
    + int empty = list_empty(&anon_vma->head);
    + int last_root_user = 0;
    + int root_empty = 0;
    +
    + /*
    + * The refcount on a non-root anon_vma got dropped. Drop
    + * the refcount on the root and check if we need to free it.
    + */
    + if (empty && anon_vma != root) {
    + last_root_user = atomic_dec_and_test(&root->external_refcount);
    + root_empty = list_empty(&root->head);
    + }
    + anon_vma_unlock(anon_vma);
    +
    + if (empty) {
    + anon_vma_free(anon_vma);
    + if (root_empty && last_root_user)
    + anon_vma_free(root);
    + }
    + }
    +}
    +#endif
    +
    #ifdef CONFIG_MIGRATION
    /*
    * rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
    Index: linux-2.6/mm/migrate.c
    ===================================================================
    --- linux-2.6.orig/mm/migrate.c
    +++ linux-2.6/mm/migrate.c
    @@ -639,7 +639,7 @@ static int unmap_and_move(new_page_t get
    * exist when the page is remapped later
    */
    anon_vma = page_anon_vma(page);
    - atomic_inc(&anon_vma->external_refcount);
    + get_anon_vma(anon_vma);
    }
    }

    @@ -682,12 +682,8 @@ skip_unmap:
    rcu_unlock:

    /* Drop an anon_vma reference if we took one */
    - if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
    - int empty = list_empty(&anon_vma->head);
    - anon_vma_unlock(anon_vma);
    - if (empty)
    - anon_vma_free(anon_vma);
    - }
    + if (anon_vma)
    + drop_anon_vma(anon_vma);

    if (rcu_locked)
    rcu_read_unlock();



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