mm/memcg: revise the using condition of lock_page_lruvec function series

lock_page_lruvec() and its variants are safe to use under the same
conditions as commit_charge(): add lock_page_memcg() to the comment.

Polished with Hugh Dickins' suggestions, thanks!

Link: https://lkml.kernel.org/r/1608614453-10739-1-git-send-email-alex.shi@linux.alibaba.comSigned-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1346,10 +1346,11 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct page *page)
  * lock_page_lruvec - lock and return lruvec for a given page.
  * @page: the page
  *
- * This series functions should be used in either conditions:
- * PageLRU is cleared or unset
- * or page->_refcount is zero
- * or page is locked.
+ * These functions are safe to use under any of the following conditions:
+ * - page locked
+ * - PageLRU cleared
+ * - lock_page_memcg()
+ * - page->_refcount is zero
  */
 struct lruvec *lock_page_lruvec(struct page *page)
 {