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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: References: <20260427-page_mapped-v1-0-e89c3592c74c@kernel.org> <20260427-page_mapped-v1-2-e89c3592c74c@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260427-page_mapped-v1-2-e89c3592c74c@kernel.org> X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D7D95C0009 X-Rspam-User: X-Stat-Signature: upc9yo46aoff9knufgqf1gqjesxdzrmw X-HE-Tag: 1777294844-144849 X-HE-Meta: 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 9GtsvCq0 0KpNbUfSp5BG+q5saIogXrGx6rMQIgB4+MfD/gzwoQornt/pgxqAhs5U0kJsPMcq98WAcuIWcw9iHRH1FrIo3b3GQk/B5JvYuyNENKkdWMOecXccji5LRhgc+n0Jt8mJoPAdx2O0xSRh2ELKL7xXn6G7JDrdzj4ZngS0SzYxSNeRGu9gG1zPk5wItphZJD4Ag9EQI6kBGhRAYrssyCwsUr/WM2O/X05/+j85YCYItl/KN1PXuAOnVH+7UaJ66v9tFOtQcj22aTZVcs6IjHZg0pTFgkbzJxyX/JgQc6btITKGxbrr4NOo2rzSB1AXOauYfFtD55LfE3WVA0uSghmVRve8Rht+YyuIcrfzB Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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)