Subject: fix refcount bug in anon_vma code

From: Rik van Riel <riel@redhat.com>

With the new anon_vma code we take a refcount on the root anon_vma.
However, the root anon_vma does not have a refcount on itself, so
we should not try to do a drop on itself when it is being unlinked.

Just got merged in the -mm kernel.

Fixes bug 602739

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Dave Young <hidave.darkstar@gmail.com>
---

diff --git a/mm/rmap.c b/mm/rmap.c
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -275,7 +275,8 @@ static void anon_vma_unlink(struct anon_
 
 	if (empty) {
 		/* We no longer need the root anon_vma */
-		drop_anon_vma(anon_vma->root);
+		if (anon_vma->root != anon_vma)
+			drop_anon_vma(anon_vma->root);
 		anon_vma_free(anon_vma);
 	}
 }