mm: 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-by: Larry Woodman <lwoodman@redhat.com>
Acked-by: Larry Woodman <lwoodman@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 24 insertions, 10 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index eb9f6806ed51..da6037c261f1 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -325,7 +325,7 @@ static void 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->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)
diff --git a/mm/migrate.c b/mm/migrate.c
index 1855f869917d..5208fa1d9712 100644
--- a/mm/migrate.c
+++ b/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)
diff --git a/mm/mmap.c b/mm/mmap.c
index f5db18decc2e..fb89360a2120 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -506,6 +506,7 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
         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->vm_end);
                 }
         }
 
+        /*
+         * 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->vm_end);
                 __insert_vm_struct(mm, insert);
         }
 
+        if (anon_vma)
+                anon_vma_unlock(anon_vma);
         if (mapping)
                 spin_unlock(&mapping->i_mmap_lock);
 
@@ -2470,23 +2484,23 @@ 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();
         }
 }
@@ -2577,7 +2591,7 @@ 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.
@@ -2588,10 +2602,10 @@ static void vm_unlock_anon_vma(struct anon_vma *anon_vma)
                  *
                  * 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);
         }