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Subject[PATCH 05/13] 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 use this new refcount in the migration code to avoid a long
RCU read side section and convert page_lock_anon_vma() over to use
refcounts.

This later 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 | 7 +++++++
mm/ksm.c | 9 +--------
mm/migrate.c | 17 ++++++-----------
mm/rmap.c | 45 ++++++++++++++++++++++++++++++---------------
4 files changed, 44 insertions(+), 34 deletions(-)
Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -100,6 +100,13 @@ static inline void anon_vma_merge(struct
unlink_anon_vmas(next);
}

+struct anon_vma *anon_vma_get(struct page *page);
+static inline void anon_vma_put(struct anon_vma *anon_vma)
+{
+ if (atomic_dec_and_test(&anon_vma->ref))
+ anon_vma_free(anon_vma);
+}
+
/*
* 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
@@ -323,14 +323,7 @@ static void hold_anon_vma(struct rmap_it

static void drop_anon_vma(struct rmap_item *rmap_item)
{
- struct anon_vma *anon_vma = rmap_item->anon_vma;
-
- if (atomic_dec_and_lock(&anon_vma->ref, &anon_vma->lock)) {
- int empty = list_empty(&anon_vma->head);
- spin_unlock(&anon_vma->lock);
- if (empty)
- anon_vma_free(anon_vma);
- }
+ anon_vma_put(rmap_item->anon_vma);
}

/*
Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c
+++ linux-2.6/mm/migrate.c
@@ -545,7 +545,7 @@ static int unmap_and_move(new_page_t get
int rc = 0;
int *result = NULL;
struct page *newpage = get_new_page(page, private, &result);
- int rcu_locked = 0;
+ struct anon_vma *anon_vma = NULL;
int charge = 0;
struct mem_cgroup *mem = NULL;

@@ -601,10 +601,8 @@ static int unmap_and_move(new_page_t get
* File Caches may use write_page() or lock_page() in migration, then,
* just care Anon page here.
*/
- if (PageAnon(page)) {
- rcu_read_lock();
- rcu_locked = 1;
- }
+ if (PageAnon(page))
+ anon_vma = anon_vma_get(page);

/*
* Corner case handling:
@@ -622,10 +620,7 @@ static int unmap_and_move(new_page_t get
if (!PageAnon(page) && page_has_private(page)) {
/*
* Go direct to try_to_free_buffers() here because
- * a) that's what try_to_release_page() would do anyway
- * b) we may be under rcu_read_lock() here, so we can't
- * use GFP_KERNEL which is what try_to_release_page()
- * needs to be effective.
+ * that's what try_to_release_page() would do anyway
*/
try_to_free_buffers(page);
goto rcu_unlock;
@@ -643,8 +638,8 @@ skip_unmap:
if (rc)
remove_migration_ptes(page, page);
rcu_unlock:
- if (rcu_locked)
- rcu_read_unlock();
+ if (anon_vma)
+ anon_vma_put(anon_vma);
uncharge:
if (!charge)
mem_cgroup_end_migration(mem, page, newpage);
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -66,11 +66,18 @@ 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->ref, 1);
+
+ return anon_vma;
}

void anon_vma_free(struct anon_vma *anon_vma)
{
+ VM_BUG_ON(atomic_read(&anon_vma->ref));
kmem_cache_free(anon_vma_cachep, anon_vma);
}

@@ -149,7 +156,7 @@ int anon_vma_prepare(struct vm_area_stru

spin_unlock(&anon_vma->lock);
if (unlikely(allocated)) {
- anon_vma_free(allocated);
+ anon_vma_put(allocated);
anon_vma_chain_free(avc);
}
}
@@ -230,7 +237,7 @@ int anon_vma_fork(struct vm_area_struct
return 0;

out_error_free_anon_vma:
- anon_vma_free(anon_vma);
+ anon_vma_put(anon_vma);
out_error:
unlink_anon_vmas(vma);
return -ENOMEM;
@@ -247,13 +254,11 @@ static void anon_vma_unlink(struct anon_

spin_lock(&anon_vma->lock);
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->ref);
+ empty = list_empty(&anon_vma->head);
spin_unlock(&anon_vma->lock);

if (empty)
- anon_vma_free(anon_vma);
+ anon_vma_put(anon_vma);
}

void unlink_anon_vmas(struct vm_area_struct *vma)
@@ -286,11 +291,11 @@ void __init anon_vma_init(void)

/*
* Getting a lock on a stable anon_vma from a page off the LRU is
- * tricky: page_lock_anon_vma rely on RCU to guard against the races.
+ * tricky: page_lock_anon_vma relies on RCU to guard against the races.
*/
-struct anon_vma *page_lock_anon_vma(struct page *page)
+struct anon_vma *anon_vma_get(struct page *page)
{
- struct anon_vma *anon_vma;
+ struct anon_vma *anon_vma = NULL;
unsigned long anon_mapping;

rcu_read_lock();
@@ -302,23 +307,33 @@ again:
goto out;

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
- spin_lock(&anon_vma->lock);
+ if (!atomic_inc_not_zero(&anon_vma->ref))
+ anon_vma = NULL;

if (page_rmapping(page) != anon_vma) {
- spin_unlock(&anon_vma->lock);
+ anon_vma_put(anon_vma);
goto again;
}

- return anon_vma;
out:
rcu_read_unlock();
- return NULL;
+ return anon_vma;
+}
+
+struct anon_vma *page_lock_anon_vma(struct page *page)
+{
+ struct anon_vma *anon_vma = anon_vma_get(page);
+
+ if (anon_vma)
+ spin_lock(&anon_vma->lock);
+
+ return anon_vma;
}

void page_unlock_anon_vma(struct anon_vma *anon_vma)
{
spin_unlock(&anon_vma->lock);
- rcu_read_unlock();
+ anon_vma_put(anon_vma);
}

/*



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