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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 27, 2026 at 01:43:15PM +0200, David Hildenbrand (Arm) 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? But
> likely we would not want to use folios here longterm, as we don't really
> need folio information.
> 
> 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.

Well, if we GUPped the page, it must have been mapped at some point ;-)

Anyway, I approve of this patch; it lets us defer the hard decisions and
work until we know what we really need.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>