lkml.org 
[lkml]   [2010]   [May]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    Subject[PATCH 4/5] always lock the root (oldest) anon_vma
    Subject: always lock the root (oldest) anon_vma

    Always (and only) lock the root (oldest) anon_vma whenever we do something in an
    anon_vma. The recently introduced anon_vma scalability is due to the rmap code
    scanning only the VMAs that need to be scanned. Many common operations still
    took the anon_vma lock on the root anon_vma, so always taking that lock is not
    expected to introduce any scalability issues.

    However, always taking the same lock does mean we only need to take one lock,
    which means rmap_walk on pages from any anon_vma in the vma is excluded from
    occurring during an munmap, expand_stack or other operation that needs to
    exclude rmap_walk and similar functions.

    Also add the proper locking to vma_adjust.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    ---
    v3:
    - fix locking inversion in vma_adjust, spotted by Andrea
    - mm_take_all locks needs to use the bitflag in the root anon_vma,
    since that is the one that gets locked (Andrea Arcangeli)
    v2:
    - conditionally take the anon_vma lock in vma_adjust, like introduced
    in 252c5f94d944487e9f50ece7942b0fbf659c5c31 (with a proper comment)

    include/linux/rmap.h | 8 ++++----
    mm/ksm.c | 2 +-
    mm/migrate.c | 2 +-
    mm/mmap.c | 30 ++++++++++++++++++++++--------
    4 files changed, 28 insertions(+), 14 deletions(-)

    Index: linux-2.6.34/include/linux/rmap.h
    ===================================================================
    --- linux-2.6.34.orig/include/linux/rmap.h
    +++ linux-2.6.34/include/linux/rmap.h
    @@ -104,24 +104,24 @@ static inline void vma_lock_anon_vma(str
    {
    struct anon_vma *anon_vma = vma->anon_vma;
    if (anon_vma)
    - spin_lock(&anon_vma->lock);
    + spin_lock(&anon_vma->root->lock);
    }

    static inline void vma_unlock_anon_vma(struct vm_area_struct *vma)
    {
    struct anon_vma *anon_vma = vma->anon_vma;
    if (anon_vma)
    - spin_unlock(&anon_vma->lock);
    + spin_unlock(&anon_vma->root->lock);
    }

    static inline void anon_vma_lock(struct anon_vma *anon_vma)
    {
    - spin_lock(&anon_vma->lock);
    + spin_lock(&anon_vma->root->lock);
    }

    static inline void anon_vma_unlock(struct anon_vma *anon_vma)
    {
    - spin_unlock(&anon_vma->lock);
    + spin_unlock(&anon_vma->root->lock);
    }

    /*
    Index: linux-2.6.34/mm/ksm.c
    ===================================================================
    --- linux-2.6.34.orig/mm/ksm.c
    +++ linux-2.6.34/mm/ksm.c
    @@ -325,7 +325,7 @@ static void drop_anon_vma(struct rmap_it
    {
    struct anon_vma *anon_vma = rmap_item->anon_vma;

    - if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->lock)) {
    + 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)
    Index: linux-2.6.34/mm/mmap.c
    ===================================================================
    --- linux-2.6.34.orig/mm/mmap.c
    +++ linux-2.6.34/mm/mmap.c
    @@ -506,6 +506,7 @@ int vma_adjust(struct vm_area_struct *vm
    struct vm_area_struct *importer = NULL;
    struct address_space *mapping = NULL;
    struct prio_tree_root *root = NULL;
    + struct anon_vma *anon_vma = NULL;
    struct file *file = vma->vm_file;
    long adjust_next = 0;
    int remove_next = 0;
    @@ -578,6 +579,17 @@ again: remove_next = 1 + (end > next->
    }
    }

    + /*
    + * When changing only vma->vm_end, we don't really need anon_vma
    + * lock. This is a fairly rare case by itself, but the anon_vma
    + * lock may be shared between many sibling processes. Skipping
    + * the lock for brk adjustments makes a difference sometimes.
    + */
    + if (vma->anon_vma && (insert || importer || start != vma->vm_start)) {
    + anon_vma = vma->anon_vma;
    + anon_vma_lock(anon_vma);
    + }
    +
    if (root) {
    flush_dcache_mmap_lock(mapping);
    vma_prio_tree_remove(vma, root);
    @@ -617,6 +629,8 @@ again: remove_next = 1 + (end > next->
    __insert_vm_struct(mm, insert);
    }

    + if (anon_vma)
    + anon_vma_unlock(anon_vma);
    if (mapping)
    spin_unlock(&mapping->i_mmap_lock);

    @@ -2466,23 +2480,23 @@ static DEFINE_MUTEX(mm_all_locks_mutex);

    static void vm_lock_anon_vma(struct mm_struct *mm, struct anon_vma *anon_vma)
    {
    - if (!test_bit(0, (unsigned long *) &anon_vma->head.next)) {
    + if (!test_bit(0, (unsigned long *) &anon_vma->root->head.next)) {
    /*
    * The LSB of head.next can't change from under us
    * because we hold the mm_all_locks_mutex.
    */
    - spin_lock_nest_lock(&anon_vma->lock, &mm->mmap_sem);
    + spin_lock_nest_lock(&anon_vma->root->lock, &mm->mmap_sem);
    /*
    * We can safely modify head.next after taking the
    - * anon_vma->lock. If some other vma in this mm shares
    + * anon_vma->root->lock. If some other vma in this mm shares
    * the same anon_vma we won't take it again.
    *
    * No need of atomic instructions here, head.next
    * can't change from under us thanks to the
    - * anon_vma->lock.
    + * anon_vma->root->lock.
    */
    if (__test_and_set_bit(0, (unsigned long *)
    - &anon_vma->head.next))
    + &anon_vma->root->head.next))
    BUG();
    }
    }
    @@ -2573,7 +2587,7 @@ out_unlock:

    static void vm_unlock_anon_vma(struct anon_vma *anon_vma)
    {
    - if (test_bit(0, (unsigned long *) &anon_vma->head.next)) {
    + if (test_bit(0, (unsigned long *) &anon_vma->root->head.next)) {
    /*
    * The LSB of head.next can't change to 0 from under
    * us because we hold the mm_all_locks_mutex.
    @@ -2584,10 +2598,10 @@ static void vm_unlock_anon_vma(struct an
    *
    * No need of atomic instructions here, head.next
    * can't change from under us until we release the
    - * anon_vma->lock.
    + * anon_vma->root->lock.
    */
    if (!__test_and_clear_bit(0, (unsigned long *)
    - &anon_vma->head.next))
    + &anon_vma->root->head.next))
    BUG();
    anon_vma_unlock(anon_vma);
    }
    Index: linux-2.6.34/mm/migrate.c
    ===================================================================
    --- linux-2.6.34.orig/mm/migrate.c
    +++ linux-2.6.34/mm/migrate.c
    @@ -682,7 +682,7 @@ 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->lock)) {
    + 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)


    \
     
     \ /
      Last update: 2010-05-26 21:45    [W:0.031 / U:30.556 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site