[PATCH mm-new] mm/migrate: move NR_KERNEL_FILE_PAGES on cross-node migration

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

NR_KERNEL_FILE_PAGES counts page cache folios of mappings flagged
AS_KERNEL_FILE (currently the btrfs btree inode).  It is a per-node
counter: filemap_add_folio() adds to folio_pgdat() of the new folio and
filemap_unaccount_folio() subtracts from folio_pgdat() of the folio
being removed.

__folio_migrate_mapping() does not move it when such a folio is
migrated to another node, so the source node keeps the count forever
and the destination node underflows once the folio is freed.  btree
folios are migratable through btree_migrate_folio(), so demotion,
DAMON-based migration (DAMOS_MIGRATE_{HOT,COLD}) or memory offlining
all hit this.

Reproducer, on a QEMU guest with two memory tiers (node0 is DRAM with
all CPUs, node1 is a CPU-less slower node described via HMAT) and
/sys/kernel/mm/numa/demotion_enabled set:

  1. mount a btrfs and create 20000 empty files, so that btree folios
     are cached on node0;
  2. echo 2G > /sys/devices/system/node/node0/reclaim, which demotes
     them to node1;
  3. umount and drop caches.

nr_kernel_file_pages in /sys/devices/system/node/nodeN/vmstat after
each step:

                  without this patch     with this patch
                   node0     node1       node0     node1
  1. cached         1868         0        1868         0
  2. demoted        1868         0          36      1832
  3. dropped        1832         0 (*)       0         0

  (*) the counter has underflowed to -1832; negative values read as 0.

The global value in /proc/vmstat is 0 after step 3 in both cases.

Move it together with the other node-keyed page cache counters.  It is
a node-only counter, so use the node accessor, matching how it is
accounted elsewhere.  Interrupts are disabled here, as for the zone
counters below.

This was pointed out by Shivank while reviewing the NR_FILE_THPS fix
for the same function.

Fixes: e3a9ac4e866e ("mm: add vmstat for kernel_file pages")
Suggested-by: Shivank Garg <shivankg@amd.com>
Assisted-by: LLM
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 mm/migrate.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 99e3026e3126..f829c5017c96 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -705,6 +705,12 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 			mod_lruvec_state(old_lruvec, NR_FILE_THPS, -nr);
 			mod_lruvec_state(new_lruvec, NR_FILE_THPS, nr);
 		}
+		if (test_bit(AS_KERNEL_FILE, &mapping->flags)) {
+			__mod_node_page_state(oldzone->zone_pgdat,
+					      NR_KERNEL_FILE_PAGES, -nr);
+			__mod_node_page_state(newzone->zone_pgdat,
+					      NR_KERNEL_FILE_PAGES, nr);
+		}
 #ifdef CONFIG_SWAP
 		if (folio_test_swapcache(folio)) {
 			mod_lruvec_state(old_lruvec, NR_SWAPCACHE, -nr);
-- 
2.34.1