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Subject[PATCH 07/28] extend KSM refcounts to the anon_vma root
From: Rik van Riel <riel@redhat.com>

KSM reference counts can cause an anon_vma to exist after the processe
it belongs to have already exited. Because the anon_vma lock now lives
in the root anon_vma, we need to ensure that the root anon_vma stays
around until after all the "child" anon_vmas have been freed.

The obvious way to do this is to have a "child" anon_vma take a
reference to the root in anon_vma_fork. When the anon_vma is freed
at munmap or process exit, we drop the refcount in anon_vma_unlink
and possibly free the root anon_vma.

The KSM anon_vma reference count function also needs to be modified
to deal with the possibility of freeing 2 levels of anon_vma. The
easiest way to do this is to break out the KSM magic and make it
generic.

When compiling without CONFIG_KSM, this code is compiled out.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100526154124.04607d04@annuminas.surriel.com>
---
v2:
- merge with -mm and the compaction code
- improve the anon_vma refcount comment in anon_vma_fork with the
refcount lifetime
include/linux/rmap.h | 15 +++++++++++++++
mm/ksm.c | 17 ++++++-----------
mm/migrate.c | 10 +++-------
mm/rmap.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
4 files changed, 68 insertions(+), 19 deletions(-)
Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -81,6 +81,13 @@ static inline int anonvma_external_refco
{
return atomic_read(&anon_vma->external_refcount);
}
+
+static inline void get_anon_vma(struct anon_vma *anon_vma)
+{
+ atomic_inc(&anon_vma->external_refcount);
+}
+
+void drop_anon_vma(struct anon_vma *);
#else
static inline void anonvma_external_refcount_init(struct anon_vma *anon_vma)
{
@@ -90,6 +97,14 @@ static inline int anonvma_external_refco
{
return 0;
}
+
+static inline void get_anon_vma(struct anon_vma *anon_vma)
+{
+}
+
+static inline void drop_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/ksm.c
===================================================================
--- linux-2.6.orig/mm/ksm.c
+++ linux-2.6/mm/ksm.c
@@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_it
struct anon_vma *anon_vma)
{
rmap_item->anon_vma = anon_vma;
- atomic_inc(&anon_vma->external_refcount);
+ get_anon_vma(anon_vma);
}

-static void drop_anon_vma(struct rmap_item *rmap_item)
+static void ksm_drop_anon_vma(struct rmap_item *rmap_item)
{
struct anon_vma *anon_vma = rmap_item->anon_vma;

- if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
- int empty = list_empty(&anon_vma->head);
- anon_vma_unlock(anon_vma);
- if (empty)
- anon_vma_free(anon_vma);
- }
+ drop_anon_vma(anon_vma);
}

/*
@@ -415,7 +410,7 @@ 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.
*/
- drop_anon_vma(rmap_item);
+ ksm_drop_anon_vma(rmap_item);

down_read(&mm->mmap_sem);
if (ksm_test_exit(mm))
@@ -470,7 +465,7 @@ static void remove_node_from_stable_tree
ksm_pages_sharing--;
else
ksm_pages_shared--;
- drop_anon_vma(rmap_item);
+ ksm_drop_anon_vma(rmap_item);
rmap_item->address &= PAGE_MASK;
cond_resched();
}
@@ -558,7 +553,7 @@ static void remove_rmap_item_from_tree(s
else
ksm_pages_shared--;

- drop_anon_vma(rmap_item);
+ ksm_drop_anon_vma(rmap_item);
rmap_item->address &= PAGE_MASK;

} else if (rmap_item->address & UNSTABLE_FLAG) {
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -235,6 +235,12 @@ int anon_vma_fork(struct vm_area_struct
* lock any of the anon_vmas in this anon_vma tree.
*/
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.
+ */
+ get_anon_vma(anon_vma->root);
/* Mark this anon_vma as the one where our new (COWed) pages go. */
vma->anon_vma = anon_vma;
anon_vma_chain_link(vma, avc, anon_vma);
@@ -264,8 +270,11 @@ static void anon_vma_unlink(struct anon_
empty = list_empty(&anon_vma->head) && !anonvma_external_refcount(anon_vma);
anon_vma_unlock(anon_vma);

- if (empty)
+ if (empty) {
+ /* We no longer need the root anon_vma */
+ drop_anon_vma(anon_vma->root);
anon_vma_free(anon_vma);
+ }
}

void unlink_anon_vmas(struct vm_area_struct *vma)
@@ -1382,6 +1391,40 @@ 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
+ * 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)
+{
+ if (atomic_dec_and_lock(&anon_vma->external_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->external_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);
+ }
+ }
+}
+#endif
+
#ifdef CONFIG_MIGRATION
/*
* rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c
+++ linux-2.6/mm/migrate.c
@@ -639,7 +639,7 @@ static int unmap_and_move(new_page_t get
* exist when the page is remapped later
*/
anon_vma = page_anon_vma(page);
- atomic_inc(&anon_vma->external_refcount);
+ get_anon_vma(anon_vma);
}
}

@@ -682,12 +682,8 @@ skip_unmap:
rcu_unlock:

/* Drop an anon_vma reference if we took one */
- if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->root->lock)) {
- int empty = list_empty(&anon_vma->head);
- anon_vma_unlock(anon_vma);
- if (empty)
- anon_vma_free(anon_vma);
- }
+ if (anon_vma)
+ drop_anon_vma(anon_vma);

if (rcu_locked)
rcu_read_unlock();



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