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b=d5W1HDLzSBMx3frw2niVk0GR2O9vcnpmuDh3nVZ09DxzHczZ5uzY2u225TKsjUTua bZnCGRvMT7WmR5K5UJUVsUJk/Rt1oUCOQozAkKZYIbZ5sd1vuDPYRD+oXzRT//KPRH DvrTY8DWFMcDHceh3SgIZQCVBHw+KKQ9PO9czG3ta2WIyETUS5IG2NxNpHIC333463 3fjyC5hLQVzbEqVqV9OP0tgNsDBH5aj4jjeUmOX8aBQDVWuDn+VTfFHZSlo2clBzd6 ViGv4WtWdnCqQdApp6geo/GRYIvEtQRzpZfwufEoU/00iPUg7GvvN2ngvmfIGhopfE ecrZkicAFaTMA== Message-ID: <5f61a5fd-ab38-4635-aab8-5de515c4015a@kernel.org> Date: Fri, 20 Feb 2026 17:50:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Per-process page size To: Matthew Wilcox Cc: Dev Jain , lsf-pc@lists.linux-foundation.org, ryan.roberts@arm.com, catalin.marinas@arm.com, will@kernel.org, ardb@kernel.org, hughd@google.com, baolin.wang@linux.alibaba.com, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260217145026.3880286-1-dev.jain@arm.com> <2e68ef61-dcf2-46b2-913f-14980a104faf@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: mj5no18ikynhrpjf6wzp8tfzkdwxiq5z X-Rspamd-Server: rspam11 X-Rspam-User: X-Rspamd-Queue-Id: A6A6540002 X-HE-Tag: 1771606251-990258 X-HE-Meta: U2FsdGVkX1/X+tPL9IEAFqdalLVxtlvRqm1QbIr7rIAtrw7YXmCZlJzhOe0ow5KYXKR2o09y/lVd70tO2/EqQWWUrs4AEibi6QIbiXjIGYy3OJwx5svsmgyTbYkeYvni0Fwh4T4gSvV7GveNWsJtqt+XNjU31BzNBwxa6rw5opvgzH+4j0H5baZI4afC7jeN6NT02OSJP5suNhFJuH22vDgDeTKZEpIC9VlEanEAjkkLdmWm0la9PMEbYwr7SUD321ukjP7Rg62fh2ZZQdJiG/gIO080mVbv4eJillAyxUvtrehpn5I7OM9rdzN/XkdQZ9klaIkdL4cz2nJX8Y6pZgJaUvL1XX6YsOSFUpcFRRZw6kGQg5OQ853REW9R4LDkj0sJFedH+4YB7IlUtzrYB/hxM+XYM4B8a+uRgljiL6oSwChwHMHsA3HAm9UD0Ae6XzmdLg2UiOw0jrPxE8GrNxna2yrcHJZ5d7hoLoLuLdtliDzqNx3MOzabGxcTc2IAK174QTp2KqScv5sUMw3RZJZBtCunkeqRrTz3g2ZbwUhtua67M6/A8qPv6dQHdpVo8+RhBqEts4bwstwYvQWSgVAv066dFUtwAtZzR3smSnynzZH+7hCigz6x1UggJgQ9DuHXgKoOZ/lnYscf0sFX3lCQLxsxIZY8aCsyDH8h5DPTwAnFMlhE+v22qIbX4ELEZ2ofLUXq8B2hZmOK7PcIz99FLNy0Rlt4z8mvJew60nYL/80UxzdY6dUO9fezwOCXUlAkzdny0RzJJIt3RWlMSS1dBujDtE0sxse4B9obUDiljrimffsxOQKNvC/UInIWxSQGCRKZOlcyOcX6Sfy4UT/LFGIE4Fl4xFv4JxW+0fr9e0m7PyLWV0rQJSKwZa/jagTDtpPrQlsauFO0k8GP8D3ts3DsUgmPmZpcTDR6ZWPPRpgMZgEt3EfRsA3vGQUMvoP6BX7XwHFNcUTnfPt jjsjyx4j cyDL44vZRzvwBMiz//XfJNqvsWM+gI44d4ataOCjRRV4ny0a8RXHKXhdapXSeMJRsULjGusPxQjuqkWUDsXn8sq2UZ8JWVBdp//fb6g3SmaD/+/Ek6YW13HwZ2mMWu4Ryu7SYyFI1liE9qGgLSrqA+lgTk2nQmgatRub8LA7xqmsVCeWL7cSMUt+rJq9NWf5GN56QLZHEwkgffVnwkQUTpPJFh5oa18S9YiBs3XKh1grAARk2nPzB0Ts0X//JS6vm74gv X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/20/26 05:49, Matthew Wilcox wrote: > On Tue, Feb 17, 2026 at 04:30:59PM +0100, David Hildenbrand (Arm) wrote: >> In a private conversation I also raised that some situations might make it >> impossible/hard to drop+re-read. >> >> One example I cam up with if a folio is simply long-term R/O pinned. But I >> am also not quite sure how mlock might interfere here. >> >> So yes, I think the page cache is likely the one of the most >> problematic/messy thing to handle. > > So what if we convert to max-supported-order the first time somebody > calls mmap on a given file? Most files are never mmaped, so it won't > affect them. Yes! > And files that are mmaped are generally not written to. Well, let's say many mmaped files are not written to. :) > So there should not be much in the page cache for the common case. You'd assume many files to either get mmaped or read/written, yes. > And if no pages from the file have been mmaped yet, they cannot be pinned > or mlocked. Is there some other way for someone to block a page from getting evicted from the pagecache? We have this memfd_pin_folios() thing, but I don't think we have something comparable for ordinary pagecache files. ... putting them into a pipe and never reading from the pipe maybe (I assume that's what splice() does, but not sure if it actually places the pages in there or whether it creates a copy first)? But yes, doing the conversion on first mmap() should handle mlock+gup. -- Cheers, David