mm: memcg: only check for PageSwapCache when uncharging anon

Only anon pages that are uncharged at the time of the last page table
mapping vanishing may be in swapcache.

When shmem pages, file pages, swap-freed anon pages, or just migrated
pages are uncharged, they are known for sure to be not in swapcache.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Wanpeng Li <liwp.linux@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3093,8 +3093,6 @@ void mem_cgroup_uncharge_cache_page(struct page *page)
 {
 	VM_BUG_ON(page_mapped(page));
 	VM_BUG_ON(page->mapping);
-	if (PageSwapCache(page))
-		return;
 	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE, false);
 }
 
@@ -3159,8 +3157,6 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 	if (!swapout) /* this was a swap cache but the swap is unused ! */
 		ctype = MEM_CGROUP_CHARGE_TYPE_DROP;
 
-	if (PageSwapCache(page))
-		return;
 	memcg = __mem_cgroup_uncharge_common(page, ctype, false);
 
 	/*
@@ -3350,11 +3346,10 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 		unused = oldpage;
 	}
 	anon = PageAnon(used);
-	if (!PageSwapCache(unused))
-		__mem_cgroup_uncharge_common(unused,
-					     anon ? MEM_CGROUP_CHARGE_TYPE_ANON
-					     : MEM_CGROUP_CHARGE_TYPE_CACHE,
-					     true);
+	__mem_cgroup_uncharge_common(unused,
+				     anon ? MEM_CGROUP_CHARGE_TYPE_ANON
+				     : MEM_CGROUP_CHARGE_TYPE_CACHE,
+				     true);
 	css_put(&memcg->css);
 	/*
 	 * We disallowed uncharge of pages under migration because mapcount