Re: [PATCH v2 2/3] mm: add vmstat for cgroup uncharged pages

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Aug 15, 2025 at 04:40:32PM -0700, Boris Burkov wrote:
> Uncharged pages are tricky to track by their essential "uncharged"
> nature. To maintain good accounting, introduce a vmstat counter tracking
> all uncharged pages. Since this is only meaningful when cgroups are
> configured, only expose the counter when CONFIG_MEMCG is set.
> 
> Confirmed that these work as expected at a high level by mounting a
> btrfs using AS_UNCHARGED for metadata pages, and seeing the counter rise
> with fs usage then go back to a minimal level after drop_caches and
> finally down to 0 after unmounting the fs.
> 
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: Boris Burkov <boris@bur.io>
> ---
>  include/linux/mmzone.h |  3 +++
>  mm/filemap.c           | 17 +++++++++++++++++
>  mm/vmstat.c            |  3 +++
>  3 files changed, 23 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 0c5da9141983..f6d885c97e99 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -245,6 +245,9 @@ enum node_stat_item {
>  	NR_HUGETLB,
>  #endif
>  	NR_BALLOON_PAGES,
> +#ifdef CONFIG_MEMCG
> +	NR_UNCHARGED_FILE_PAGES,
> +#endif
>  	NR_VM_NODE_STAT_ITEMS
>  };
>  
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6046e7f27709..cd5af44a838c 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -146,6 +146,19 @@ static void page_cache_delete(struct address_space *mapping,
>  	mapping->nrpages -= nr;
>  }
>  
> +#ifdef CONFIG_MEMCG
> +static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
> +{
> +	long nr = folio_nr_pages(folio) * sign;
> +
> +	lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);

Since we expect to add this metric to memory.stat, I think we should use
mod_node_page_state() instead here.

With that you can add:

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>