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

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

On 10/26/2026 8:51 PM, Garg, Shivank wrote:
> On Mon, 2026-10-26 at 11:59 +0100, David Hildenbrand (Arm) wrote:
>> 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>
>>
> 
> filemap_unaccount_folio() also checks for AS_KERNEL_FILE, but
> __folio_migrate_mapping() does not.
> 
> Should add this as well?
> 
Good catch, thanks. NR_KERNEL_FILE_PAGES has the same problem, and I
could reproduce it by demoting btrfs btree folios to a CPU-less memory
tier. Since it was introduced separately by e3a9ac4e866e ("mm: add
vmstat for kernel_file pages") and is a node-only counter, I sent it
as a separate patch on top of this one:

https://lore.kernel.org/20261027043214.2248986-1-ridong.chen@linux.dev

-- 
Best regards
Ridong