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

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

On 10/30/2026 9:03 AM, Andrew Morton wrote:
> On Tue, 27 Oct 2026 12:32:14 +0800 Ridong Chen <ridong.chen@linux.dev> wrote:
> 
>> From: Ridong Chen <chenridong@xiaomi.com>
>>
>> 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.
>>
> 
> Thanks.
> 
> This and your "mm/migrate: move NR_FILE_THPS on cross-node migration"
> "only" address accounting errors, but a bug is a bug and these bugs
> might mess up people's memory-monitoring and NUMA-management tools.  So
> I think cc:stable.
> 
> What do people think?

Agreed, thanks.

-- 
Best regards
Ridong