+ mm-memcg-fix-swapcached-stat-accounting.patch added to mm-unstable branch

+ mm-memcg-fix-swapcached-stat-accounting.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: memcg: fix swapcached stat accounting
has been added to the -mm mm-unstable branch.  Its filename is
     mm-memcg-fix-swapcached-stat-accounting.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memcg-fix-swapcached-stat-accounting.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Hugh Dickins <hughd@google.com>
Subject: mm: memcg: fix swapcached stat accounting
Date: Sun, 4 Dec 2022 17:01:03 -0800 (PST)

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>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |    6 ++++++
 mm/vmscan.c     |    3 +--
 2 files changed, 7 insertions(+), 2 deletions(-)

--- a/mm/memcontrol.c~mm-memcg-fix-swapcached-stat-accounting
+++ a/mm/memcontrol.c
@@ -5756,6 +5756,12 @@ static int mem_cgroup_move_account(struc
 		}
 	}
 
+#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);
--- a/mm/vmscan.c~mm-memcg-fix-swapcached-stat-accounting
+++ a/mm/vmscan.c
@@ -1368,11 +1368,10 @@ static int __remove_mapping(struct addre
 	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 {
_

Patches currently in -mm which might be from hughd@google.com are

tmpfs-fix-data-loss-from-failed-fallocate.patch
mm-memcg-fix-swapcached-stat-accounting.patch