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>

1 files changed, 6 insertions, 11 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) {