+ mm-migrate-move-nr_kernel_file_pages-on-cross-node-migration.patch added to mm-new branch

+ mm-migrate-move-nr_kernel_file_pages-on-cross-node-migration.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/migrate: move NR_KERNEL_FILE_PAGES on cross-node migration
has been added to the -mm mm-new branch.  Its filename is
     mm-migrate-move-nr_kernel_file_pages-on-cross-node-migration.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-migrate-move-nr_kernel_file_pages-on-cross-node-migration.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Ridong Chen <chenridong@xiaomi.com>
Subject: mm/migrate: move NR_KERNEL_FILE_PAGES on cross-node migration
Date: Tue, 27 Oct 2026 12:32:14 +0800

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.

Link: https://lore.kernel.org/20261027043214.2248986-1-ridong.chen@linux.dev
Fixes: e3a9ac4e866e ("mm: add vmstat for kernel_file pages")
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Shivank Garg <shivankg@amd.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Assisted-by: LLM
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Boris Burkov <boris@bur.io>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
---

 mm/migrate.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/mm/migrate.c~mm-migrate-move-nr_kernel_file_pages-on-cross-node-migration
+++ a/mm/migrate.c
@@ -705,6 +705,12 @@ static int __folio_migrate_mapping(struc
 			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);
_

Patches currently in -mm which might be from chenridong@xiaomi.com are

mm-migrate-move-nr_file_thps-on-cross-node-migration.patch
mm-migrate-move-nr_kernel_file_pages-on-cross-node-migration.patch