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Subject[PATCH 10/28] mm: Make use of the anon_vma ref count
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.

We then convert page_lock_anon_vma() over to use refcounts. This is
done for each of convertion of anon_vma from spinlock to mutex.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/rmap.h | 11 ++++-
mm/ksm.c | 4 --
mm/rmap.c | 94 ++++++++++++++++++++++-----------------------------
3 files changed, 52 insertions(+), 57 deletions(-)
Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -73,7 +73,13 @@ static inline void get_anon_vma(struct a
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_str
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
unlink_anon_vmas(next);
}

+struct anon_vma *page_get_anon_vma(struct page *page);
+
/*
* rmap interfaces called when adding or removing pte of page
*/
Index: linux-2.6/mm/ksm.c
===================================================================
--- linux-2.6.orig/mm/ksm.c
+++ linux-2.6/mm/ksm.c
@@ -323,9 +323,7 @@ static void hold_anon_vma(struct rmap_it

static void ksm_put_anon_vma(struct rmap_item *rmap_item)
{
- struct anon_vma *anon_vma = rmap_item->anon_vma;
-
- put_anon_vma(anon_vma);
+ put_anon_vma(rmap_item->anon_vma);
}

/*
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -66,11 +66,24 @@ static struct kmem_cache *anon_vma_chain

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);
+ /*
+ * 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;
+ }
+
+ 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);
}

@@ -132,11 +145,6 @@ int anon_vma_prepare(struct vm_area_stru
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);
@@ -155,7 +163,7 @@ int anon_vma_prepare(struct vm_area_stru
anon_vma_unlock(anon_vma);

if (unlikely(allocated))
- anon_vma_free(allocated);
+ put_anon_vma(allocated);
if (unlikely(avc))
anon_vma_chain_free(avc);
}
@@ -248,7 +256,7 @@ int anon_vma_fork(struct vm_area_struct
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;
@@ -265,16 +273,12 @@ static void anon_vma_unlink(struct anon_

anon_vma_lock(anon_vma);
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 */
- 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)
@@ -326,9 +330,9 @@ void __init anon_vma_init(void)
* that the anon_vma pointer from page->mapping is valid if there is a
* mapcount, we can dereference the anon_vma after observing those.
*/
-struct anon_vma *page_lock_anon_vma(struct page *page)
+struct anon_vma *page_get_anon_vma(struct page *page)
{
- struct anon_vma *anon_vma;
+ struct anon_vma *anon_vma = NULL;
unsigned long anon_mapping;

rcu_read_lock();
@@ -339,17 +343,28 @@ struct anon_vma *page_lock_anon_vma(stru
goto out;

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
- anon_vma_lock(anon_vma);
- return anon_vma;
+ if (!atomic_inc_not_zero(&anon_vma->refcount))
+ anon_vma = NULL;
out:
rcu_read_unlock();
- return NULL;
+
+ return anon_vma;
+}
+
+struct anon_vma *page_lock_anon_vma(struct page *page)
+{
+ struct anon_vma *anon_vma = page_get_anon_vma(page);
+
+ if (anon_vma)
+ anon_vma_lock(anon_vma);
+
+ return anon_vma;
}

void page_unlock_anon_vma(struct anon_vma *anon_vma)
{
anon_vma_unlock(anon_vma);
- rcu_read_unlock();
+ put_anon_vma(anon_vma);
}

/*
@@ -1405,36 +1420,11 @@ 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)
-{
- 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;
-
- /*
- * 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) {
- last_root_user = atomic_dec_and_test(&root->refcount);
- root_empty = list_empty(&root->head);
- }
- anon_vma_unlock(anon_vma);
-
- if (empty) {
- anon_vma_free(anon_vma);
- if (root_empty && last_root_user)
- anon_vma_free(root);
- }
- }
+void __put_anon_vma(struct anon_vma *anon_vma)
+{
+ if (anon_vma->root != anon_vma)
+ put_anon_vma(anon_vma->root);
+ anon_vma_free(anon_vma);
}

#ifdef CONFIG_MIGRATION



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