memcg: move charge swapin under lock

While page-cache's charge/uncharge is done under page_lock(), swap-cache
isn't.  (anonymous page is charged when it's newly allocated.)

This patch moves do_swap_page()'s charge() call under lock.  I don't see
any bad problem *now* but this fix will be good for future for avoiding
unnecessary racy state.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2326,16 +2326,17 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		count_vm_event(PGMAJFAULT);
 	}
 
-	if (mem_cgroup_charge(page, mm, GFP_KERNEL)) {
+	mark_page_accessed(page);
+
+	lock_page(page);
 	delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
+
+	if (mem_cgroup_charge(page, mm, GFP_KERNEL)) {
 		ret = VM_FAULT_OOM;
+		unlock_page(page);
 		goto out;
 	}
 
-	mark_page_accessed(page);
-	lock_page(page);
-	delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
-
 	/*
 	 * Back out if somebody else already faulted in this pte.
 	 */