[PATCH] mm: fix unsafe page -> lruvec lookups with cgroup charge migration fix

[Johannes Weiner]

Better document the mem_cgroup_page_lruvec() caller requirements.

Suggested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/memcontrol.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 50f5bc55fcec..2d700fa0d7f4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1202,9 +1202,18 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
  * @page: the page
  * @pgdat: pgdat of the page
  *
- * This function is only safe when following the LRU page isolation
- * and putback protocol: the LRU lock must be held, and the page must
- * either be PageLRU() or the caller must have isolated/allocated it.
+ * NOTE: The returned lruvec is only stable if the calling context has
+ * the page->mem_cgroup pinned! This is accomplished by satisfying one
+ * of the following criteria:
+ *
+ *    a) have the @page locked
+ *    b) have an exclusive reference to @page (e.g. refcount 0)
+ *    c) hold the lru_lock and "own" the PageLRU (meaning either ensure
+ *       it's set, or be the one to hold the page in isolation)
+ *
+ * Otherwise, the page could be freed or moved out of the memcg,
+ * thereby releasing its reference on the memcg and potentially
+ * freeing it and its lruvecs in the process.
  */
 struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgdat)
 {
-- 
2.24.0