mm: simplify anon_vma refcounts

This patch changes the anon_vma refcount to be 0 when the object is free.
It does this by adding 1 ref to being in use in the anon_vma structure
(iow.  the anon_vma->head list is not empty).

This allows a simpler release scheme without having to check both the
refcount and the list as well as avoids taking a ref for each entry on the
list.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 28 insertions, 48 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 873cd9ef912c..4a8e99a0fb97 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -67,11 +67,24 @@ static struct kmem_cache *anon_vma_chain_cachep;
 
 static inline struct anon_vma *anon_vma_alloc(void)
 {
-        return kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
+        struct anon_vma *anon_vma;
+
+        anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
+        if (anon_vma) {
+                atomic_set(&anon_vma->refcount, 1);
+                /*
+                 * Initialise the anon_vma root to point to itself. If called
+                 * from fork, the root will be reset to the parents anon_vma.
+                 */
+                anon_vma->root = anon_vma;
+        }
+
+        return anon_vma;
 }
 
-void anon_vma_free(struct anon_vma *anon_vma)
+static inline void anon_vma_free(struct anon_vma *anon_vma)
 {
+        VM_BUG_ON(atomic_read(&anon_vma->refcount));
         kmem_cache_free(anon_vma_cachep, anon_vma);
 }
 
@@ -133,11 +146,6 @@ int anon_vma_prepare(struct vm_area_struct *vma)
                         if (unlikely(!anon_vma))
                                 goto out_enomem_free_avc;
                         allocated = anon_vma;
-                        /*
-                         * This VMA had no anon_vma yet.  This anon_vma is
-                         * the root of any anon_vma tree that might form.
-                         */
-                        anon_vma->root = anon_vma;
                 }
 
                 anon_vma_lock(anon_vma);
@@ -156,7 +164,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
                 anon_vma_unlock(anon_vma);
 
                 if (unlikely(allocated))
-                        anon_vma_free(allocated);
+                        put_anon_vma(allocated);
                 if (unlikely(avc))
                         anon_vma_chain_free(avc);
         }
@@ -241,9 +249,9 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
          */
         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.
+         * With 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. */
@@ -253,7 +261,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
         return 0;
 
  out_error_free_anon_vma:
-        anon_vma_free(anon_vma);
+        put_anon_vma(anon_vma);
  out_error:
         unlink_anon_vmas(vma);
         return -ENOMEM;
@@ -272,15 +280,11 @@ static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
         list_del(&anon_vma_chain->same_anon_vma);
 
         /* We must garbage collect the anon_vma if it's empty */
-        empty = list_empty(&anon_vma->head) && !atomic_read(&anon_vma->refcount);
+        empty = list_empty(&anon_vma->head);
         anon_vma_unlock(anon_vma);
 
-        if (empty) {
-                /* We no longer need the root anon_vma */
-                if (anon_vma->root != anon_vma)
-                        put_anon_vma(anon_vma->root);
-                anon_vma_free(anon_vma);
-        }
+        if (empty)
+                put_anon_vma(anon_vma);
 }
 
 void unlink_anon_vmas(struct vm_area_struct *vma)
@@ -1486,38 +1490,14 @@ int try_to_munlock(struct page *page)
                 return try_to_unmap_file(page, TTU_MUNLOCK);
 }
 
-/*
- * 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 put_anon_vma(struct anon_vma *anon_vma)
+void __put_anon_vma(struct anon_vma *anon_vma)
 {
-        BUG_ON(atomic_read(&anon_vma->refcount) <= 0);
-        if (atomic_dec_and_lock(&anon_vma->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;
+        struct anon_vma *root = anon_vma->root;
 
-                /*
-                 * 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) {
-                        BUG_ON(atomic_read(&root->refcount) <= 0);
-                        last_root_user = atomic_dec_and_test(&root->refcount);
-                        root_empty = list_empty(&root->head);
-                }
-                anon_vma_unlock(anon_vma);
+        if (root != anon_vma && atomic_dec_and_test(&root->refcount))
+                anon_vma_free(root);
 
-                if (empty) {
-                        anon_vma_free(anon_vma);
-                        if (root_empty && last_root_user)
-                                anon_vma_free(root);
-                }
-        }
+        anon_vma_free(anon_vma);
 }
 
 #ifdef CONFIG_MIGRATION