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

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

On 10/27/26 05:32, Ridong Chen 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.
> 
> Fixes: e3a9ac4e866e ("mm: add vmstat for kernel_file pages")
> Suggested-by: Shivank Garg <shivankg@amd.com>
> Assisted-by: LLM
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---
>  mm/migrate.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 99e3026e3126..f829c5017c96 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -705,6 +705,12 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>  			mod_lruvec_state(old_lruvec, NR_FILE_THPS, -nr);
>  			mod_lruvec_state(new_lruvec, NR_FILE_THPS, nr);
>  		}
> +		if (test_bit(AS_KERNEL_FILE, &mapping->flags)) {
> +			__mod_node_page_state(oldzone->zone_pgdat,
> +					      NR_KERNEL_FILE_PAGES, -nr);
> +			__mod_node_page_state(newzone->zone_pgdat,
> +					      NR_KERNEL_FILE_PAGES, nr);
> +		}
>  #ifdef CONFIG_SWAP
>  		if (folio_test_swapcache(folio)) {
>  			mod_lruvec_state(old_lruvec, NR_SWAPCACHE, -nr);

Today I learned about NR_KERNEL_FILE_PAGES :D

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

-- 
Cheers,

David