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Subject[PATCH] 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 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.

Fixes: b6038942480e ("mm: memcg: add swapcache stat for memcg v2")
Signed-off-by: Hugh Dickins <>

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

--- 6.1-rc8/mm/memcontrol.c
+++ linux/mm/memcontrol.c
@@ -5741,6 +5741,12 @@ static int mem_cgroup_move_account(struc

+ if (folio_test_swapcache(folio)) {
+ __mod_lruvec_state(from_vec, NR_SWAPCACHE, -nr_pages);
+ __mod_lruvec_state(to_vec, NR_SWAPCACHE, nr_pages);
+ }
if (folio_test_writeback(folio)) {
__mod_lruvec_state(from_vec, NR_WRITEBACK, -nr_pages);
__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
--- 6.1-rc8/mm/vmscan.c
+++ linux/mm/vmscan.c
@@ -1346,11 +1346,10 @@ 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);
put_swap_folio(folio, swap);
} else {
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