mm: extend KSM refcounts to the anon_vma root

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>
Tested-by: Larry Woodman <lwoodman@redhat.com>
Acked-by: Larry Woodman <lwoodman@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 54 insertions, 19 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index da6037c261f1..9f2acc998a37 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_item *rmap_item,
                           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 *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(struct stable_node *stable_node)
                         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(struct rmap_item *rmap_item)
                 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) {
diff --git a/mm/migrate.c b/mm/migrate.c
index 5208fa1d9712..38e7cad782f4 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -639,7 +639,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                          * 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();
diff --git a/mm/rmap.c b/mm/rmap.c
index caa48b27371b..07e9814c7a41 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -235,6 +235,12 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
          * 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 +270,12 @@ static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
         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 */
+                if (anon_vma->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 +1392,40 @@ 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():