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@xxxxxxxxxx>
---
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)
--
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