mm: memcg: fix swapcached stat accounting

I'd been worried by high "swapcached" counts in memcg OOM reports, thought
we had a problem freeing swapcache, but it was just the accounting that
was wrong.

Two issues:

1.  When __remove_mapping() removes swapcache,
   __delete_from_swap_cache() relies on memcg_data for the right counts to
   be updated; but that had already been reset by mem_cgroup_swapout(). 
   Swap those calls around - mem_cgroup_swapout() does not require the
   swapcached flag to be set.

   6.1 commit ac35a49 ("mm: multi-gen LRU: minimal
   implementation") already made a similar swap for workingset_eviction(),
   but not for this.

2.  memcg's "swapcached" count was added for memcg v2 stats, but
   displayed on OOM even for memcg v1: so mem_cgroup_move_account() ought
   to move it.

Link: https://lkml.kernel.org/r/b8b96ee0-1e1e-85f8-df97-c82a11d7cd14@google.com
Fixes: b603894 ("mm: memcg: add swapcache stat for memcg v2")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -5767,6 +5767,12 @@ static int mem_cgroup_move_account(struct page *page,
 		}
 	}
 
+#ifdef CONFIG_SWAP
+	if (folio_test_swapcache(folio)) {
+		__mod_lruvec_state(from_vec, NR_SWAPCACHE, -nr_pages);
+		__mod_lruvec_state(to_vec, NR_SWAPCACHE, nr_pages);
+	}
+#endif
 	if (folio_test_writeback(folio)) {
 		__mod_lruvec_state(from_vec, NR_WRITEBACK, -nr_pages);
 		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);

mm/vmscan.c

@@ -1368,11 +1368,10 @@ static int __remove_mapping(struct address_space *mapping, struct folio *folio,
 	if (folio_test_swapcache(folio)) {
 		swp_entry_t swap = folio_swap_entry(folio);
 
-		/* get a shadow entry before mem_cgroup_swapout() clears folio_memcg() */
 		if (reclaimed && !mapping_exiting(mapping))
 			shadow = workingset_eviction(folio, target_memcg);
-		mem_cgroup_swapout(folio, swap);
 		__delete_from_swap_cache(folio, swap, shadow);
+		mem_cgroup_swapout(folio, swap);
 		xa_unlock_irq(&mapping->i_pages);
 		put_swap_folio(folio, swap);
 	} else {