[PATCH 0/3] mm: Simplify anon_vma lifetime rules

[PATCH 0/3] mm: Simplify anon_vma lifetime rules

[Peter Zijlstra]

As per Linus' request, isolate these three patches.


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[PATCH 1/3] mm: Rename drop_anon_vma to put_anon_vma

[Peter Zijlstra]

[-- Attachment #1: peter_zijlstra-mm-rename_drop_anon_vma_to_put_anon_vma.patch --]
[-- Type: text/plain, Size: 4647 bytes --]

The normal code pattern used in the kernel is: get/put.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 include/linux/rmap.h |    4 ++--
 mm/ksm.c             |   23 +++++------------------
 mm/migrate.c         |    4 ++--
 mm/rmap.c            |    4 ++--
 4 files changed, 11 insertions(+), 24 deletions(-)

Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -87,7 +87,7 @@ static inline void get_anon_vma(struct a
 	atomic_inc(&anon_vma->external_refcount);
 }
 
-void drop_anon_vma(struct anon_vma *);
+void put_anon_vma(struct anon_vma *);
 #else
 static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
 {
@@ -102,7 +102,7 @@ static inline void get_anon_vma(struct a
 {
 }
 
-static inline void drop_anon_vma(struct anon_vma *anon_vma)
+static inline void put_anon_vma(struct anon_vma *anon_vma)
 {
 }
 #endif /* CONFIG_KSM */
Index: linux-2.6/mm/ksm.c
===================================================================
--- linux-2.6.orig/mm/ksm.c
+++ linux-2.6/mm/ksm.c
@@ -301,20 +301,6 @@ static inline int in_stable_tree(struct 
 	return rmap_item->address & STABLE_FLAG;
 }
 
-static void hold_anon_vma(struct rmap_item *rmap_item,
-			  struct anon_vma *anon_vma)
-{
-	rmap_item->anon_vma = anon_vma;
-	get_anon_vma(anon_vma);
-}
-
-static void ksm_drop_anon_vma(struct rmap_item *rmap_item)
-{
-	struct anon_vma *anon_vma = rmap_item->anon_vma;
-
-	drop_anon_vma(anon_vma);
-}
-
 /*
  * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
  * page tables after it has passed through ksm_exit() - which, if necessary,
@@ -397,7 +383,7 @@ static void break_cow(struct rmap_item *
 	 * It is not an accident that whenever we want to break COW
 	 * to undo, we also need to drop a reference to the anon_vma.
 	 */
-	ksm_drop_anon_vma(rmap_item);
+	put_anon_vma(rmap_item->anon_vma);
 
 	down_read(&mm->mmap_sem);
 	if (ksm_test_exit(mm))
@@ -466,7 +452,7 @@ static void remove_node_from_stable_tree
 			ksm_pages_sharing--;
 		else
 			ksm_pages_shared--;
-		ksm_drop_anon_vma(rmap_item);
+		put_anon_vma(rmap_item->anon_vma);
 		rmap_item->address &= PAGE_MASK;
 		cond_resched();
 	}
@@ -554,7 +540,7 @@ static void remove_rmap_item_from_tree(s
 		else
 			ksm_pages_shared--;
 
-		ksm_drop_anon_vma(rmap_item);
+		put_anon_vma(rmap_item->anon_vma);
 		rmap_item->address &= PAGE_MASK;
 
 	} else if (rmap_item->address & UNSTABLE_FLAG) {
@@ -949,7 +935,8 @@ static int try_to_merge_with_ksm_page(st
 		goto out;
 
 	/* Must get reference to anon_vma while still holding mmap_sem */
-	hold_anon_vma(rmap_item, vma->anon_vma);
+	rmap_item->anon_vma = vma->anon_vma;
+	get_anon_vma(vma->anon_vma);
 out:
 	up_read(&mm->mmap_sem);
 	return err;
Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c
+++ linux-2.6/mm/migrate.c
@@ -764,7 +764,7 @@ static int unmap_and_move(new_page_t get
 
 	/* Drop an anon_vma reference if we took one */
 	if (anon_vma)
-		drop_anon_vma(anon_vma);
+		put_anon_vma(anon_vma);
 
 uncharge:
 	if (!charge)
@@ -857,7 +857,7 @@ static int unmap_and_move_huge_page(new_
 		remove_migration_ptes(hpage, hpage);
 
 	if (anon_vma)
-		drop_anon_vma(anon_vma);
+		put_anon_vma(anon_vma);
 out:
 	unlock_page(hpage);
 
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -278,7 +278,7 @@ static void anon_vma_unlink(struct anon_
 	if (empty) {
 		/* We no longer need the root anon_vma */
 		if (anon_vma->root != anon_vma)
-			drop_anon_vma(anon_vma->root);
+			put_anon_vma(anon_vma->root);
 		anon_vma_free(anon_vma);
 	}
 }
@@ -1489,7 +1489,7 @@ int try_to_munlock(struct page *page)
  * we know we are the last user, nobody else can get a reference and we
  * can do the freeing without the lock.
  */
-void drop_anon_vma(struct anon_vma *anon_vma)
+void put_anon_vma(struct anon_vma *anon_vma)
 {
 	BUG_ON(atomic_read(&anon_vma->external_refcount) <= 0);
 	if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {


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Re: [PATCH 1/3] mm: Rename drop_anon_vma to put_anon_vma

[Rik van Riel]

On 02/17/2011 11:19 AM, Peter Zijlstra wrote:

Reviewed-by: Rik van Riel <riel@redhat.com>

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Re: [PATCH 1/3] mm: Rename drop_anon_vma to put_anon_vma

[Mel Gorman]

On Thu, Feb 17, 2011 at 05:19:49PM +0100, Peter Zijlstra wrote:
> The normal code pattern used in the kernel is: get/put.
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Acked-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Acked-by: Mel Gorman <mel@csn.ul.ie>

-- 
Mel Gorman
SUSE Labs

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[PATCH 2/3] mm: Move anon_vma ref out from under CONFIG_foo

[Peter Zijlstra]

[-- Attachment #1: peter_zijlstra-mm-move_anon_vma_ref_out_from_under_config_ksm.patch --]
[-- Type: text/plain, Size: 4890 bytes --]

We need the anon_vma refcount unconditionally to simplify the anon_vma
lifetime rules.

Acked-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 include/linux/rmap.h |   40 ++++------------------------------------
 mm/rmap.c            |   14 ++++++--------
 2 files changed, 10 insertions(+), 44 deletions(-)

Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -27,18 +27,15 @@
 struct anon_vma {
 	struct anon_vma *root;	/* Root of this anon_vma tree */
 	spinlock_t lock;	/* Serialize access to vma list */
-#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
-
 	/*
-	 * The external_refcount is taken by either KSM or page migration
-	 * to take a reference to an anon_vma when there is no
+	 * The refcount is taken on an anon_vma when there is no
 	 * guarantee that the vma of page tables will exist for
 	 * the duration of the operation. A caller that takes
 	 * the reference is responsible for clearing up the
 	 * anon_vma if they are the last user on release
 	 */
-	atomic_t external_refcount;
-#endif
+	atomic_t refcount;
+
 	/*
 	 * NOTE: the LSB of the head.next is set by
 	 * mm_take_all_locks() _after_ taking the above lock. So the
@@ -71,41 +68,12 @@ struct anon_vma_chain {
 };
 
 #ifdef CONFIG_MMU
-#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
-static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
-{
-	atomic_set(&anon_vma->external_refcount, 0);
-}
-
-static inline int anonvma_external_refcount(struct anon_vma *anon_vma)
-{
-	return atomic_read(&anon_vma->external_refcount);
-}
-
 static inline void get_anon_vma(struct anon_vma *anon_vma)
 {
-	atomic_inc(&anon_vma->external_refcount);
+	atomic_inc(&anon_vma->refcount);
 }
 
 void put_anon_vma(struct anon_vma *);
-#else
-static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
-{
-}
-
-static inline int anonvma_external_refcount(struct anon_vma *anon_vma)
-{
-	return 0;
-}
-
-static inline void get_anon_vma(struct anon_vma *anon_vma)
-{
-}
-
-static inline void put_anon_vma(struct anon_vma *anon_vma)
-{
-}
-#endif /* CONFIG_KSM */
 
 static inline struct anon_vma *page_anon_vma(struct page *page)
 {
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -272,7 +272,7 @@ static void anon_vma_unlink(struct anon_
 	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) && !anonvma_external_refcount(anon_vma);
+	empty = list_empty(&anon_vma->head) && !atomic_read(&anon_vma->refcount);
 	anon_vma_unlock(anon_vma);
 
 	if (empty) {
@@ -303,7 +303,7 @@ static void anon_vma_ctor(void *data)
 	struct anon_vma *anon_vma = data;
 
 	spin_lock_init(&anon_vma->lock);
-	anonvma_external_refcount_init(anon_vma);
+	atomic_set(&anon_vma->refcount, 0);
 	INIT_LIST_HEAD(&anon_vma->head);
 }
 
@@ -1482,7 +1482,6 @@ int try_to_munlock(struct page *page)
 		return try_to_unmap_file(page, TTU_MUNLOCK);
 }
 
-#if defined(CONFIG_KSM) || defined(CONFIG_MIGRATION)
 /*
  * 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
@@ -1491,8 +1490,8 @@ int try_to_munlock(struct page *page)
  */
 void put_anon_vma(struct anon_vma *anon_vma)
 {
-	BUG_ON(atomic_read(&anon_vma->external_refcount) <= 0);
-	if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
+	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;
@@ -1503,8 +1502,8 @@ void put_anon_vma(struct anon_vma *anon_
 		 * the refcount on the root and check if we need to free it.
 		 */
 		if (empty && anon_vma != root) {
-			BUG_ON(atomic_read(&root->external_refcount) <= 0);
-			last_root_user = atomic_dec_and_test(&root->external_refcount);
+			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);
@@ -1516,7 +1515,6 @@ void put_anon_vma(struct anon_vma *anon_
 		}
 	}
 }
-#endif
 
 #ifdef CONFIG_MIGRATION
 /*


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Re: [PATCH 2/3] mm: Move anon_vma ref out from under CONFIG_foo

[Rik van Riel]

On 02/17/2011 11:19 AM, Peter Zijlstra wrote:

Acked-by: Rik van Riel <riel@redhat.com>

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[PATCH 3/3] mm: Simplify anon_vma refcounts

[Peter Zijlstra]

[-- Attachment #1: peter_zijlstra-mm-simplify_anon_vma_refcounts.patch --]
[-- Type: text/plain, Size: 6391 bytes --]

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.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 include/linux/rmap.h |   11 +++++--
 mm/rmap.c            |   79 ++++++++++++++++++---------------------------------
 2 files changed, 37 insertions(+), 53 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/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -67,11 +67,24 @@ 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);
+		/*
+		 * 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_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);
@@ -156,7 +164,7 @@ 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);
 	}
@@ -241,9 +249,9 @@ int anon_vma_fork(struct vm_area_struct 
 	 */
 	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 
 	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_
 	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)
@@ -1482,38 +1486,11 @@ 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)
-{
-	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;
-
-		/*
-		 * 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 (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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Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

[Rik van Riel]

On 02/17/2011 11:19 AM, Peter Zijlstra wrote:

Acked-by: Rik van Riel <riel@redhat.com>

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Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

[Linus Torvalds]

On Thu, Feb 17, 2011 at 8:19 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> +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);
>  }

So this makes me nervous. It looks like recursion.

Now, I don't think we can ever get a chain of these things (because
the root should be the root of everything), but I still preferred the
older code that made that "one-level root" case explicit, and didn't
have recursion.

IOW, even though it should be entirely equivalent, I think I'd really
prefer something like

  void __put_anon_vma(struct anon_vma *anon_vma)
  {
    struct anon_vma *root = anon_vma->root;

    if (root != anon_vma && atomic_dec_and_test(&root->refcount))
      anon_vma_free(root);
    anon_vma_free(anon_vma);
  }

instead. Exactly because it makes it very clear that the "root" is a
root, and we're not doing some possibly arbitrarily deep list like the
dentry tree (which avoids recursion by open-coding its freeing as a
loop).

Hmm? (The above is obviously untested, maybe it has some stupid bug)

                   Linus

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Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

[Peter Zijlstra]

On Thu, 2011-02-17 at 10:30 -0800, Linus Torvalds wrote:
> On Thu, Feb 17, 2011 at 8:19 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >
> > +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);
> >  }
> 
> So this makes me nervous. It looks like recursion.
> 
> Now, I don't think we can ever get a chain of these things (because
> the root should be the root of everything),

Exactly.

>  but I still preferred the
> older code that made that "one-level root" case explicit, and didn't
> have recursion.
> 
> IOW, even though it should be entirely equivalent, I think I'd really
> prefer something like
> 
>   void __put_anon_vma(struct anon_vma *anon_vma)
>   {
>     struct anon_vma *root = anon_vma->root;
> 
>     if (root != anon_vma && atomic_dec_and_test(&root->refcount))
>       anon_vma_free(root);
>     anon_vma_free(anon_vma);
>   }
> 
> instead. Exactly because it makes it very clear that the "root" is a
> root, and we're not doing some possibly arbitrarily deep list like the
> dentry tree (which avoids recursion by open-coding its freeing as a
> loop).
> 
> Hmm? (The above is obviously untested, maybe it has some stupid bug)

Looks about right, I'll give it a spin. 

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Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

[Peter Zijlstra]

Subject: mm: Simplify anon_vma refcounts
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Fri, 26 Nov 2010 15:38:49 +0100

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.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 include/linux/rmap.h |   11 +++++--
 mm/rmap.c            |   78 ++++++++++++++++++---------------------------------
 2 files changed, 38 insertions(+), 51 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/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -67,11 +67,24 @@ 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);
+		/*
+		 * 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_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);
@@ -156,7 +164,7 @@ 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);
 	}
@@ -241,9 +249,9 @@ int anon_vma_fork(struct vm_area_struct 
 	 */
 	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 
 	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_
 	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)
@@ -1470,38 +1474,14 @@ 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)
-{
-	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;
+void __put_anon_vma(struct anon_vma *anon_vma)
+{
+	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


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Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

[Rik van Riel]

On 02/18/2011 08:44 AM, Peter Zijlstra wrote:
> Subject: mm: Simplify anon_vma refcounts
> From: Peter Zijlstra<a.p.zijlstra@chello.nl>
> Date: Fri, 26 Nov 2010 15:38:49 +0100
>
> 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.
>
> Reviewed-by: KAMEZAWA Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
> Acked-by: Hugh Dickins<hughd@google.com>
> Signed-off-by: Peter Zijlstra<a.p.zijlstra@chello.nl>

Acked-by: Rik van Riel <riel@redhat.com>

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Re: [PATCH 3/3] mm: Simplify anon_vma refcounts

[Mel Gorman]

On Fri, Feb 18, 2011 at 02:44:43PM +0100, Peter Zijlstra wrote:
> Subject: mm: Simplify anon_vma refcounts
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Fri, 26 Nov 2010 15:38:49 +0100
> 
> 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.
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Acked-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Acked-by: Mel Gorman <mel@csn.ul.ie>

-- 
Mel Gorman
SUSE Labs

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Re: [PATCH 0/3] mm: Simplify anon_vma lifetime rules

[Peter Zijlstra]

On Thu, 2011-02-17 at 17:19 +0100, Peter Zijlstra wrote:
> As per Linus' request, isolate these three patches.

---
 include/linux/rmap.h |   45 ++++++---------------------
 mm/ksm.c             |   23 +++-----------
 mm/migrate.c         |    4 +-
 mm/rmap.c            |   83 +++++++++++++++++----------------------------------
 4 files changed, 46 insertions(+), 109 deletions(-)

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