mm: fix page_lock_anon_vma leaving mutex locked

On one machine I've been getting hangs, a page fault's anon_vma_prepare()
waiting in anon_vma_lock(), other processes waiting for that page's lock.

This is a replay of last year's f1819427 "mm: fix hang on
anon_vma->root->lock".

The new page_lock_anon_vma() places too much faith in its refcount: when
it has acquired the mutex_trylock(), it's possible that a racing task in
anon_vma_alloc() has just reallocated the struct anon_vma, set refcount
to 1, and is about to reset its anon_vma->root.

Fix this by saving anon_vma->root, and relying on the usual page_mapped()
check instead of a refcount check: if page is still mapped, the anon_vma
is still ours; if page is not still mapped, we're no longer interested.
Signed-off-by: NHugh Dickins <hughd@google.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -405,6 +405,7 @@ struct anon_vma *page_get_anon_vma(struct page *page)
 struct anon_vma *page_lock_anon_vma(struct page *page)
 {
 	struct anon_vma *anon_vma = NULL;
+	struct anon_vma *root_anon_vma;
 	unsigned long anon_mapping;
 
 	rcu_read_lock();
@@ -415,13 +416,15 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
 		goto out;
 
 	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
-	if (mutex_trylock(&anon_vma->root->mutex)) {
+	root_anon_vma = ACCESS_ONCE(anon_vma->root);
+	if (mutex_trylock(&root_anon_vma->mutex)) {
 		/*
-		 * If we observe a !0 refcount, then holding the lock ensures
-		 * the anon_vma will not go away, see __put_anon_vma().
+		 * If the page is still mapped, then this anon_vma is still
+		 * its anon_vma, and holding the mutex ensures that it will
+		 * not go away, see __put_anon_vma().
 		 */
-		if (!atomic_read(&anon_vma->refcount)) {
-			anon_vma_unlock(anon_vma);
+		if (!page_mapped(page)) {
+			mutex_unlock(&root_anon_vma->mutex);
 			anon_vma = NULL;
 		}
 		goto out;