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Howlett" , 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() Message-Id: <20260427051758.e1b714a4b567917971920eaa@linux-foundation.org> In-Reply-To: <20260427-page_mapped-v1-2-e89c3592c74c@kernel.org> References: <20260427-page_mapped-v1-0-e89c3592c74c@kernel.org> <20260427-page_mapped-v1-2-e89c3592c74c@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 0B700100006 X-Stat-Signature: 8r1kicyk6r3tp1t6peqp5cjochx8ffmt X-Rspam-User: X-HE-Tag: 1777292280-211962 X-HE-Meta: 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 03TTGC5f fItv7ud4IM4E7UBZNXZxdJYrGIW/Jl20YIgAvfRQ3khf9H9/5LUsYiQUE5PTyfgA6p2ivZmAMGxgv6aXWPyeejregCRfzuUpB0HFxtTm8GCLf2R6oPc6o+R+Q3iCFKeFVa0x/L/eonr0iz/ULSVnpGn6VP43oMFg4Jk1ZaGxQN5EZHgU1RpXU5Zb8/VpPe3uMFLYjyfT6Dc0S3iugDF/mvILvsFx4eCmLMyeUuwtugCq/MvLAemuK+n0lm1YF4GydxGwvHWJ42XNU8/iRXkV5am0d5LfSXgPr8rzRlHLeMeDs5XfX5Xj34r9Ak0LHUzq4N2Vy Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 27 Apr 2026 13:43:15 +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. > > Signed-off-by: David Hildenbrand (Arm) > --- > 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.