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

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

On Tue, Aug 05, 2025 at 05:11:49PM -0700, Boris Burkov wrote:
> If cgroups are configured into the kernel, then uncharged pages can only
> come from filemap_add_folio_nocharge. Track such uncharged folios in
> vmstat so that they are accounted for.
> 
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: Boris Burkov <boris@bur.io>
> ---
>  include/linux/mmzone.h |  3 +++
>  mm/filemap.c           | 18 ++++++++++++++++++
>  mm/vmstat.c            |  3 +++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 283913d42d7b..a945dec65371 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -241,6 +241,9 @@ 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 ccc9cfb4d418..0a258b4a9246 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -146,6 +146,22 @@ 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;
> +
> +	if (!folio_memcg(folio))
> +		__lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);

From filemap_add_folio_nocharge(), this function is called without
preemption disabled, so you will get lockdep warning from following
chain:

__lruvec_stat_mod_folio -> __mod_node_page_state ->
preempt_disable_nested -> lockdep_assert_preemption_disabled().