memcg swap: use mem_cgroup_uncharge_swap()

That stuff __mem_cgroup_commit_charge_swapin() does with a swap entry, it
has a name and even a declaration: just use mem_cgroup_uncharge_swap().

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2855,24 +2855,7 @@ __mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *memcg,
 	 */
 	if (do_swap_account && PageSwapCache(page)) {
 		swp_entry_t ent = {.val = page_private(page)};
-		struct mem_cgroup *swap_memcg;
+		mem_cgroup_uncharge_swap(ent);
-		unsigned short id;
-
-		id = swap_cgroup_record(ent, 0);
-		rcu_read_lock();
-		swap_memcg = mem_cgroup_lookup(id);
-		if (swap_memcg) {
-			/*
-			 * This recorded memcg can be obsolete one. So, avoid
-			 * calling css_tryget
-			 */
-			if (!mem_cgroup_is_root(swap_memcg))
-				res_counter_uncharge(&swap_memcg->memsw,
-						     PAGE_SIZE);
-			mem_cgroup_swap_statistics(swap_memcg, false);
-			mem_cgroup_put(swap_memcg);
-		}
-		rcu_read_unlock();
 	}
 	/*
 	 * At swapin, we may charge account against cgroup which has no tasks.