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

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

On Fri, Aug 15, 2025 at 05:48:33PM -0700, Shakeel Butt wrote:
> 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 

*never*

> 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>
>