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    SubjectRe: [PATCH 4/5] always lock the root (oldest) anon_vma
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    On Wed, 2010-05-26 at 15:40 -0400, Rik van Riel wrote:
    > 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>

    Tested and Acked-by: Larry Woodman <lwoodman@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)
    >
    > --
    > To unsubscribe, send a message with 'unsubscribe linux-mm' in
    > the body to majordomo@kvack.org. For more info on Linux MM,
    > see: http://www.linux-mm.org/ .
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