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/include/linux/rmap.h b/include/linux/rmap.h
index fd56111a52c1ca4e02c2626f5e6387da7f7bff9f..830e65dc01ee1f264e715632f2f99fd95c7bd529 100644 (file)
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -73,7 +73,13 @@ static inline void get_anon_vma(struct anon_vma *anon_vma)
        atomic_inc(&anon_vma->refcount);
 }
 
-void put_anon_vma(struct anon_vma *);
+void __put_anon_vma(struct anon_vma *anon_vma);
+
+static inline void put_anon_vma(struct anon_vma *anon_vma)
+{
+       if (atomic_dec_and_test(&anon_vma->refcount))
+               __put_anon_vma(anon_vma);
+}
 
 static inline struct anon_vma *page_anon_vma(struct page *page)
 {
@@ -116,7 +122,6 @@ void unlink_anon_vmas(struct vm_area_struct *);
 int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *);
 int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
 void __anon_vma_link(struct vm_area_struct *);
-void anon_vma_free(struct anon_vma *);
 
 static inline void anon_vma_merge(struct vm_area_struct *vma,
                                  struct vm_area_struct *next)
@@ -125,6 +130,8 @@ static inline void anon_vma_merge(struct vm_area_struct *vma,
        unlink_anon_vmas(next);
 }
 
+struct anon_vma *page_get_anon_vma(struct page *page);
+
 /*
  * rmap interfaces called when adding or removing pte of page
  */

diff --git a/mm/rmap.c b/mm/rmap.c
index 873cd9ef912c3b72013af094fe2f6c365abfc77f..4a8e99a0fb972de9b16c96a196dd78f15370ed35 100644 (file)
--- 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