Re: [PATCH] mm/migrate: move NR_FILE_THPS on cross-node migration

[Zi Yan]

On 25 Oct 2026, at 7:02, Ridong Chen wrote:

> From: Ridong Chen <chenridong@xxxxxxxxxx>
>
> 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.
>
> Fixes: d68eccad3706 ("mm/filemap: Allow large folios to be added to the page cache")
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> ---
> Note: prepared with help from an LLM-based assistant (Kiro, claude-opus-5.5).
>
>  mm/migrate.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
LGTM. Thanks.

Acked-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi