mm: memcontrol: update mem_cgroup_page_lruvec() documentation

Commit 7512102c ("memcg: fix GPF when cgroup removal races with last
exit") added a pc->mem_cgroup reset into mem_cgroup_page_lruvec() to
prevent a crash where an anon page gets uncharged on unmap, the memcg is
released, and then the final LRU isolation on free dereferences the
stale pc->mem_cgroup pointer.

But since commit 0a31bc97 ("mm: memcontrol: rewrite uncharge API"),
pages are only uncharged AFTER that final LRU isolation, which
guarantees the memcg's lifetime until then.  pc->mem_cgroup now only
needs to be reset for swapcache readahead pages.

Update the comment and callsite requirements accordingly.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1262,9 +1262,13 @@ struct lruvec *mem_cgroup_zone_lruvec(struct zone *zone,
 }
 
 /**
- * mem_cgroup_page_lruvec - return lruvec for adding an lru page
+ * mem_cgroup_page_lruvec - return lruvec for isolating/putting an LRU page
  * @page: the page
  * @zone: zone 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.
  */
 struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone)
 {
@@ -1282,13 +1286,9 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone)
 	memcg = pc->mem_cgroup;
 
 	/*
-	 * Surreptitiously switch any uncharged offlist page to root:
-	 * an uncharged page off lru does nothing to secure
-	 * its former mem_cgroup from sudden removal.
-	 *
-	 * Our caller holds lru_lock, and PageCgroupUsed is updated
-	 * under page_cgroup lock: between them, they make all uses
-	 * of pc->mem_cgroup safe.
+	 * Swapcache readahead pages are added to the LRU - and
+	 * possibly migrated - before they are charged.  Ensure
+	 * pc->mem_cgroup is sane.
 	 */
 	if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup)
 		pc->mem_cgroup = memcg = root_mem_cgroup;