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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/25/26 12:02, Ridong Chen wrote:
> From: Ridong Chen <chenridong@xiaomi.com>
> 
> 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@xiaomi.com>
> ---
> Note: prepared with help from an LLM-based assistant (Kiro, claude-opus-5.5).
> 
>  mm/migrate.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 7bdcdb57652f..d55f576a2579 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -700,6 +700,10 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>  				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);
>  		}

Looking at filemap_unaccount_folio(), I was (once again) surprise about the
different handling.

Key is that we might be migration aonymous folios in the swapcache.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>


-- 
Cheers,

David