From: Hugh Dickins <hughd@google.com>
Date: Sun, 4 Dec 2022 17:01:03 -0800
Subject: mm: memcg: fix swapcached stat accounting
Git-commit: c449deb2b99ff2458214ed4a3526277bc9e40757
Patch-mainline: v6.2-rc1
References: bsc#1209804

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 ac35a4902374 ("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: b6038942480e ("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>
Acked-by: Michal Koutný <mkoutny@suse.com>
[ mkoutny: folio->page backport, context]
---
 mm/memcontrol.c |    6 ++++++
 mm/vmscan.c     |    2 +-
 2 files changed, 7 insertions(+), 1 deletion(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5614,6 +5614,12 @@ static int mem_cgroup_move_account(struc
 		}
 	}
 
+#ifdef CONFIG_SWAP
+	if (PageSwapCache(page)) {
+		__mod_lruvec_state(from_vec, NR_SWAPCACHE, -nr_pages);
+		__mod_lruvec_state(to_vec, NR_SWAPCACHE, nr_pages);
+	}
+#endif
 	if (PageWriteback(page)) {
 		__mod_lruvec_state(from_vec, NR_WRITEBACK, -nr_pages);
 		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1225,10 +1225,10 @@ static int __remove_mapping(struct addre
 
 	if (PageSwapCache(page)) {
 		swp_entry_t swap = { .val = page_private(page) };
-		mem_cgroup_swapout(page, swap);
 		if (reclaimed && !mapping_exiting(mapping))
 			shadow = workingset_eviction(page, target_memcg);
 		__delete_from_swap_cache(page, swap, shadow);
+		mem_cgroup_swapout(page, swap);
 		xa_unlock_irqrestore(&mapping->i_pages, flags);
 		put_swap_page(page, swap);
 	} else {