mm: memcontrol: remove unnecessary PCG_MEM memory charge flag

PCG_MEM is a remnant from an earlier version of 0a31bc97c80c ("mm:
memcontrol: rewrite uncharge API"), used to tell whether migration cleared
a charge while leaving pc->mem_cgroup valid and PCG_USED set.  But in the
final version, mem_cgroup_migrate() directly uncharges the source page,
rendering this distinction unnecessary.  Remove it.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/page_cgroup.h

@@ -4,7 +4,6 @@
 enum {
 	/* flags for mem_cgroup */
 	PCG_USED = 0x01,	/* This page is charged to a memcg */
-	PCG_MEM = 0x02,		/* This page holds a memory charge */
 };
 
 struct pglist_data;

mm/memcontrol.c

@@ -2614,7 +2614,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
 	 *   have the page locked
 	 */
 	pc->mem_cgroup = memcg;
-	pc->flags = PCG_USED | PCG_MEM;
+	pc->flags = PCG_USED;
 
 	if (lrucare)
 		unlock_page_lru(page, isolated);
@@ -6156,8 +6156,6 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
 	if (!PageCgroupUsed(pc))
 		return;
 
-	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
-
 	if (lrucare)
 		lock_page_lru(oldpage, &isolated);