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

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

[Andrew Morton]

The patch titled
     Subject: mm/migrate: move NR_FILE_THPS on cross-node migration
has been added to the -mm mm-new branch.  Its filename is
     mm-migrate-move-nr_file_thps-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_file_thps-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_FILE_THPS on cross-node migration
Date: Sun, 25 Oct 2026 19:02:09 +0800

When a page cache folio is migrated to another node,
__folio_migrate_mapping() moves the per-node and per-memcg counters
that are keyed off the folio's node: NR_FILE_PAGES, NR_SHMEM,
NR_SHMEM_THPS, NR_SWAPCACHE and NR_FILE_DIRTY.  NR_FILE_THPS is not
moved, although __filemap_add_folio() charges it to the source node and
filemap_unaccount_folio() takes it off whatever node the folio lives on
when it is removed.

Commit 0b52c420350e ("mm: fix shmem THP counters on migration") added
the NR_SHMEM_THPS case but not its file-backed counterpart.  Now that
ext4, xfs, btrfs and erofs can all have PMD-sized page cache folios,
migrating them via NUMA balancing, mbind(), migratepages, demotion or
memory offlining leaks FileHugePages on the source node and underflows
it on the destination, in both /sys/devices/system/node/nodeN/meminfo
and memory.numa_stat.  The global value is unaffected, which is why this
went unnoticed.

Reproducer, on a 2-node QEMU guest with THP set to "always":

  1. read a 256M ext4 file through a MADV_HUGEPAGE mapping while bound
     to node 0, so its page cache is 128 PMD-sized folios on node 0;
  2. move the mapping to node 1 with mbind(MPOL_MF_MOVE);
  3. unmap the file and drop its page cache.

nr_file_hugepages in /sys/devices/system/node/nodeN/vmstat after each
step (in units of PMD-sized folios):

                    without this patch     with this patch
                     node0     node1       node0     node1
  1. cached           128         0         128         0
  2. migrated         128         0           0       128
  3. dropped          128         0 (*)       0         0

  (*) the counter has underflowed to -128; negative values read as 0.

Without the patch the 128 folios stay accounted to node 0 forever, and
every further round adds another 128.  The same happens to the per-memcg
per-node counter: after step 3, memory.stat of the cgroup shows
"file_thp 0", while memory.numa_stat still shows
"file_thp N0=268435456 N1=0".  With the patch both are 0.

Move NR_FILE_THPS together with NR_FILE_PAGES, using the same condition
filemap_unaccount_folio() uses to take it off, so that THPs in the swap
cache are left alone.

Link: https://lore.kernel.org/20261025110209.4105994-1-ridong.chen@linux.dev
Fixes: d68eccad3706 ("mm/filemap: Allow large folios to be added to the page cache")
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
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: Matthew Wilcox (Oracle) <willy@infradead.org>
---

 mm/migrate.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/mm/migrate.c~mm-migrate-move-nr_file_thps-on-cross-node-migration
+++ a/mm/migrate.c
@@ -700,6 +700,10 @@ static int __folio_migrate_mapping(struc
 				mod_lruvec_state(old_lruvec, NR_SHMEM_THPS, -nr);
 				mod_lruvec_state(new_lruvec, NR_SHMEM_THPS, nr);
 			}
+		} else if (!folio_test_swapbacked(folio) &&
+			   folio_test_pmd_mappable(folio)) {
+			mod_lruvec_state(old_lruvec, NR_FILE_THPS, -nr);
+			mod_lruvec_state(new_lruvec, NR_FILE_THPS, nr);
 		}
 #ifdef CONFIG_SWAP
 		if (folio_test_swapcache(folio)) {
_

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

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