Re: [PATCH 2/3] bpf: arena: use page_ref_count() instead of page_mapped() in arena_free_pages()

[Alexei Starovoitov]

On Mon, Apr 27, 2026 at 1:18 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 27 Apr 2026 13:43:15 +0200 "David Hildenbrand (Arm)" <david@xxxxxxxxxx> wrote:
>
> > Pages that BPF arena code maps are allocated through
> > bpf_map_alloc_pages(), which does not allocate folios but pages.
> >
> > In the future, pages will not have a mapcount, only folios will.
> > Converting the code to use folios and rely on folio_mapped() sounds like
> > the wrong approach.
> >
> > Should BPF arena code allocate folios and use folio_mapped() here?

no

> But
> > likely we would not want to use folios here longterm, as we don't really
> > need folio information.

exactly

> > Hard to tell. But in the meantime, we can simply use the page refcount
> > instead, as a heuristic whether the page might be mapped to user space
> > and we would want to try zapping it, so we can get rid of page_mapped().
> >
> > Page allocation will give us a page with a refcount of 1. Any user space
> > mapping adds a page reference. While there can be references from other
> > subsystems (e.g., GUP), in the common case for this test here relying on
> > the page count is good enough.

makes sense to me.

> >
> > Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> > ---
> >  kernel/bpf/arena.c | 2 +-
>
> BPF maintainers will probably want to carry this in the BPF tree.
> That's fine - please go ahead and add it.  I'll carry a duplicate in
> mm.git so it compiles.

We cannot carry the same patch in 2 trees.
Sooner or later it will create problems for linux-next
and issues during merge window if more changes
are done in the same area.
The only way to share a patch between trees is to
create a stable branch and pull it into 2 trees
then sha will be the same,
but mm tree has its own way of doing things,
so this patch needs to stay in mm only and if no one
should be touching adjacent lines :(