memcg: remove PCG_FILE_MAPPED

With the new lock scheme for updating memcg's page stat, we don't need a
flag PCG_FILE_MAPPED which was duplicated information of page_mapped().

[[e-mail]: cosmetic fix]
[[e-mail]: add comment to MEM_CGROUP_CHARGE_TYPE_MAPPED case in __mem_cgroup_uncharge_common()]
Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Acked-by: Greg Thelen <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: KOSAKI Motohiro <[e-mail]>
Cc: Ying Han <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page_cgroup.h

···
 	PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */
 	PCG_USED, /* this object is in use. */
 	PCG_MIGRATION, /* under page migration */
-	/* flags for mem_cgroup and file and I/O status */
-	PCG_FILE_MAPPED, /* page is accounted as "mapped" */
 	__NR_PCG_FLAGS,
 };
 
···
 TESTPCGFLAG(Used, USED)
 CLEARPCGFLAG(Used, USED)
 SETPCGFLAG(Used, USED)
-
-SETPCGFLAG(FileMapped, FILE_MAPPED)
-CLEARPCGFLAG(FileMapped, FILE_MAPPED)
-TESTPCGFLAG(FileMapped, FILE_MAPPED)
 
 SETPCGFLAG(Migration, MIGRATION)
 CLEARPCGFLAG(Migration, MIGRATION)

mm/memcontrol.c

···
 
 	switch (idx) {
 	case MEMCG_NR_FILE_MAPPED:
-		if (val > 0)
-			SetPageCgroupFileMapped(pc);
-		else if (!page_mapped(page))
-			ClearPageCgroupFileMapped(pc);
 		idx = MEM_CGROUP_STAT_FILE_MAPPED;
 		break;
 	default:
···
 
 	move_lock_mem_cgroup(from, &flags);
 
-	if (PageCgroupFileMapped(pc)) {
+	if (!anon && page_mapped(page)) {
 		/* Update mapped_file data for mem_cgroup */
 		preempt_disable();
 		__this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
···
 
 	switch (ctype) {
 	case MEM_CGROUP_CHARGE_TYPE_MAPPED:
+		/*
+		 * Generally PageAnon tells if it's the anon statistics to be
+		 * updated; but sometimes e.g. mem_cgroup_uncharge_page() is
+		 * used before page reached the stage of being marked PageAnon.
+		 */
 		anon = true;
 		/* fallthrough */
 	case MEM_CGROUP_CHARGE_TYPE_DROP: