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>
diff --git a/mm/rmap.c b/mm/rmap.c
index 873cd9e..4a8e99a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -67,11 +67,24 @@
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 @@
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 @@
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 @@
*/
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 @@
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 @@
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 @@
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
