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 f18194275c39 "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: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index ba58ca36fc90..6bada99cd61c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -405,6 +405,7 @@ out:
 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;