1 files changed, 15 insertions, 7 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 3a39b518a653..0eb463ea88dd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -352,6 +352,11 @@ void __init anon_vma_init(void)
  * The page might have been remapped to a different anon_vma or the anon_vma
  * returned may already be freed (and even reused).
  *
+ * In case it was remapped to a different anon_vma, the new anon_vma will be a
+ * child of the old anon_vma, and the anon_vma lifetime rules will therefore
+ * ensure that any anon_vma obtained from the page will still be valid for as
+ * long as we observe page_mapped() [ hence all those page_mapped() tests ].
+ *
  * All users of this function must be very careful when walking the anon_vma
  * chain and verify that the page in question is indeed mapped in it
  * [ something equivalent to page_mapped_in_vma() ].
@@ -405,6 +410,7 @@ 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 +421,15 @@ 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 anon_vma_free().
                  */
-                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;
@@ -1014,7 +1022,7 @@ void do_page_add_anon_rmap(struct page *page,
                 return;
 
         VM_BUG_ON(!PageLocked(page));
-        VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
+        /* address might be in next vma when migration races vma_adjust */
         if (first)
                 __page_set_anon_rmap(page, vma, address, exclusive);
         else
@@ -1709,7 +1717,7 @@ void hugepage_add_anon_rmap(struct page *page,
 
         BUG_ON(!PageLocked(page));
         BUG_ON(!anon_vma);
-        BUG_ON(address < vma->vm_start || address >= vma->vm_end);
+        /* address might be in next vma when migration races vma_adjust */
         first = atomic_inc_and_test(&page->_mapcount);
         if (first)
                 __hugepage_set_anon_rmap(page, vma, address, 0);