memcg: fix race in file_mapped accounting

Presently, memcg's FILE_MAPPED accounting has following race with
move_account (happens at rmdir()).

increment page->mapcount (rmap.c)
mem_cgroup_update_file_mapped() move_account()
lock_page_cgroup()
check page_mapped() if
page_mapped(page)>1 {
FILE_MAPPED -1 from old memcg
FILE_MAPPED +1 to old memcg
}
.....
overwrite pc->mem_cgroup
unlock_page_cgroup()
lock_page_cgroup()
FILE_MAPPED + 1 to pc->mem_cgroup
unlock_page_cgroup()

Then,
old memcg (-1 file mapped)
new memcg (+2 file mapped)

This happens because move_account see page_mapped() which is not guarded
by lock_page_cgroup(). This patch adds FILE_MAPPED flag to page_cgroup
and move account information based on it. Now, all checks are synchronous
with lock_page_cgroup().

Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Reviewed-by: Balbir Singh <[e-mail]>
Reviewed-by: Daisuke Nishimura <[e-mail]>
Cc: Andrea Righi <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page_cgroup.h

···
 	PCG_CACHE, /* charged as cache */
 	PCG_USED, /* this object is in use. */
 	PCG_ACCT_LRU, /* page has been accounted for */
+	PCG_FILE_MAPPED, /* page is accounted as "mapped" */
 };
 
 #define TESTPCGFLAG(uname, lname)			\
···
 CLEARPCGFLAG(AcctLRU, ACCT_LRU)
 TESTPCGFLAG(AcctLRU, ACCT_LRU)
 TESTCLEARPCGFLAG(AcctLRU, ACCT_LRU)
+
+
+SETPCGFLAG(FileMapped, FILE_MAPPED)
+CLEARPCGFLAG(FileMapped, FILE_MAPPED)
+TESTPCGFLAG(FileMapped, FILE_MAPPED)
 
 static inline int page_cgroup_nid(struct page_cgroup *pc)
 {

mm/memcontrol.c

···
 
 	lock_page_cgroup(pc);
 	mem = pc->mem_cgroup;
-	if (!mem)
-		goto done;
-
-	if (!PageCgroupUsed(pc))
+	if (!mem || !PageCgroupUsed(pc))
 		goto done;
 
 	/*
 	 * Preemption is already disabled. We can use __this_cpu_xxx
 	 */
-	__this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED], val);
+	if (val > 0) {
+		__this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
+		SetPageCgroupFileMapped(pc);
+	} else {
+		__this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
+		ClearPageCgroupFileMapped(pc);
+	}
 
 done:
 	unlock_page_cgroup(pc);
···
 static void __mem_cgroup_move_account(struct page_cgroup *pc,
 	struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
 {
-	struct page *page;
-
 	VM_BUG_ON(from == to);
 	VM_BUG_ON(PageLRU(pc->page));
 	VM_BUG_ON(!PageCgroupLocked(pc));
 	VM_BUG_ON(!PageCgroupUsed(pc));
 	VM_BUG_ON(pc->mem_cgroup != from);
 
-	page = pc->page;
-	if (page_mapped(page) && !PageAnon(page)) {
+	if (PageCgroupFileMapped(pc)) {
 		/* Update mapped_file data for mem_cgroup */
 		preempt_disable();
 		__this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);