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Date: Mon, 27 Apr 2026 05:17:58 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>, Rich Felker
 <dalias@libc.org>, John Paul Adrian Glaubitz
 <glaubitz@physik.fu-berlin.de>, Alexei Starovoitov <ast@kernel.org>, Daniel
 Borkmann <daniel@iogearbox.net>, Andrii Nakryiko <andrii@kernel.org>,
 Martin KaFai Lau <martin.lau@linux.dev>, Eduard Zingerman
 <eddyz87@gmail.com>, Kumar Kartikeya Dwivedi <memxor@gmail.com>, Song Liu
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 <liam@infradead.org>, linux-sh@vger.kernel.org,
 linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/3] bpf: arena: use page_ref_count() instead of
 page_mapped() in arena_free_pages()
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On Mon, 27 Apr 2026 13:43:15 +0200 "David Hildenbrand (Arm)" <david@kernel.org> 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.
> 
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
>  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.