mm: fix hang on anon_vma->root->lock

After several hours, kbuild tests hang with anon_vma_prepare() spinning on
a newly allocated anon_vma's lock - on a box with CONFIG_TREE_PREEMPT_RCU=y
(which makes this very much more likely, but it could happen without).

The ever-subtle page_lock_anon_vma() now needs a further twist: since
anon_vma_prepare() and anon_vma_fork() are liable to change the ->root
of a reused anon_vma structure at any moment, page_lock_anon_vma()
needs to check page_mapped() again before succeeding, otherwise
page_unlock_anon_vma() might address a different root->lock.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 3 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 87b9e8ad4509..f6f0d2dda2ea 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -316,7 +316,7 @@ void __init anon_vma_init(void)
  */
 struct anon_vma *page_lock_anon_vma(struct page *page)
 {
-        struct anon_vma *anon_vma;
+        struct anon_vma *anon_vma, *root_anon_vma;
         unsigned long anon_mapping;
 
         rcu_read_lock();
@@ -327,8 +327,21 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
                 goto out;
 
         anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
-        anon_vma_lock(anon_vma);
-        return anon_vma;
+        root_anon_vma = ACCESS_ONCE(anon_vma->root);
+        spin_lock(&root_anon_vma->lock);
+
+        /*
+         * If this page is still mapped, then its anon_vma cannot have been
+         * freed.  But if it has been unmapped, we have no security against
+         * the anon_vma structure being freed and reused (for another anon_vma:
+         * SLAB_DESTROY_BY_RCU guarantees that - so the spin_lock above cannot
+         * corrupt): with anon_vma_prepare() or anon_vma_fork() redirecting
+         * anon_vma->root before page_unlock_anon_vma() is called to unlock.
+         */
+        if (page_mapped(page))
+                return anon_vma;
+
+        spin_unlock(&root_anon_vma->lock);
 out:
         rcu_read_unlock();
         return NULL;