[PATCH v3] mm: Fix shmem THP counters on migration

[Jan Glauber]

The per node numa_stat values for shmem don't change on
page migration for THP:

  grep shmem /sys/fs/cgroup/machine.slice/.../memory.numa_stat:

    shmem N0=1092616192 N1=10485760
    shmem_thp N0=1092616192 N1=10485760

  migratepages 9181 0 1:

    shmem N0=0 N1=1103101952
    shmem_thp N0=1092616192 N1=10485760

Fix that by updating shmem_thp counters likewise to shmem counters
on page migration.

Signed-off-by: Jan Glauber <jglauber@xxxxxxxxxxxxxxxx>
---
v3: Use folio_test_pmd_mappable instead of folio_test_transhuge
v2: Fix compile error
---
 mm/migrate.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 01cac26a3127..6ef8b1dea2e6 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -492,6 +492,11 @@ int folio_migrate_mapping(struct address_space *mapping,
 		if (folio_test_swapbacked(folio) && !folio_test_swapcache(folio)) {
 			__mod_lruvec_state(old_lruvec, NR_SHMEM, -nr);
 			__mod_lruvec_state(new_lruvec, NR_SHMEM, nr);
+
+			if (folio_test_pmd_mappable(folio)) {
+				__mod_lruvec_state(old_lruvec, NR_SHMEM_THPS, -nr);
+				__mod_lruvec_state(new_lruvec, NR_SHMEM_THPS, nr);
+			}
 		}
 #ifdef CONFIG_SWAP
 		if (folio_test_swapcache(folio)) {
-- 
2.25.1