we have to check if OLDPAGE/NEWPAGE is a valid page after commit().
8. LRU
- Each memcg has its own private LRU. Now, it's handling is under global
+ Each memcg has its own private LRU. Now, its handling is under global
VM's control (means that it's handled under global zone->lru_lock).
Almost all routines around memcg's LRU is called by global LRU's
list management functions under zone->lru_lock().