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b=UWgvlFeEPgbZsomKQU1NvUIqK9fYWIkSSFCZHKAujA8KMLt8SSs6oMNOc2uIQPKib uEH7FKwnZcYGq+Z5VDNECI06wwfFScwuZ592Ebe8nEoIQj6ruwaOfLiLGKHqYIB0oH d1uIgkKv4jK7+8++MofJdtFvbV/xlunr1UjAD3MYlrLntzIOA8Tz8LX51EvL0ev4HL XhajvbQRN3wQZIB1kbvnDqPFHHuBbnVT+ke69lMC5yQ5fCErx4M7nL5q2JBvDQWLsx KStuO1h5ulmIRoiwJBTkn+5YcEgRwrz5jfBBK2bo8sxTsDo3ZTZO8sBmue58hiCzeT OBPdUMlA59PQA== Message-ID: <3b758468-9985-49b8-948a-e5837decf52d@kernel.org> Date: Mon, 22 Dec 2025 15:18:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment To: Kiryl Shutsemau , Muchun Song , Matthew Wilcox Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Andrew Morton , Usama Arif , Frank van der Linden References: <20251218150949.721480-1-kas@kernel.org> <20251218150949.721480-3-kas@kernel.org> <430d554c-840f-4813-b715-5191d74571a0@linux.dev> <4ctnfkw5zqneume6px2wqgdgusfdricr4uuonv7bfjheknrt2g@fdb5ceiiar4q> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <4ctnfkw5zqneume6px2wqgdgusfdricr4uuonv7bfjheknrt2g@fdb5ceiiar4q> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: pqg73sb55j5uzzu5sxzhi4fhpq4q6ieu X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 15CA140011 X-HE-Tag: 1766413116-296490 X-HE-Meta: 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 2JCEgcF1 y5c6DrqPFKa9oLr1dGBv76IGb32Xc63QqK/lCV0HEMYl2GK5oZOf3t8g7pgp0c/95nI6iivZt/1V5FC3IaLCqXtOih+rk7y0Az/loL6Sr1d3C6zEyBLSgw54sxuLIuYWeopCzcL98B0G9w/lFwAPUR5vrHbR1RQIlG3w8GsyuQIWIi0GS/VcI5G9jKjkc0B82G+pYuK+0+GIBRLybJXAvz1mbK5OXSfOK2OGsPdgNrpywndckckgKdYkPfHMT6r6nwUNGRHbxk2sQLay5BHxo+PWlJDiIr0rdTBYW 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 12/22/25 15:02, Kiryl Shutsemau wrote: > On Mon, Dec 22, 2025 at 04:34:40PM +0800, Muchun Song wrote: >> >> >> On 2025/12/18 23:09, Kiryl Shutsemau wrote: >>> The upcoming changes in compound_head() require memmap to be naturally >>> aligned to the maximum folio size. >>> >>> Add a warning if it is not. >>> >>> A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the >>> kernel is still likely to be functional if this strict check fails. >> >> Different architectures default to 2 MB alignment (mainly to >> enable huge mappings), which only accommodates folios up to >> 128 MB. Yet 1 GB huge pages are still fairly common, so >> validating 16 GB (MAX_FOLIO_SIZE) alignment seems likely to >> miss the most frequent case. > > I don't follow. 16 GB check is more strict that anything smaller. > How can it miss the most frequent case? > >> I’m concerned that this might plant a hidden time bomb: it >> could detonate at any moment in later code, silently triggering >> memory corruption or similar failures. Therefore, I don’t >> think a WARNING is a good choice. > > We can upgrade it BUG_ON(), but I want to understand your logic here > first. Definitely no BUG_ON(). I would assume this is something we would find early during testing, so even a VM_WARN_ON_ONCE() should be good enough? This smells like a possible problem, though, as soon as some architecture wants to increase the folio size. What would be the expected step to ensure the alignment is done properly? But OTOH, as I raised Willy's work will make all of that here obsolete either way, so maybe not worth worrying about that case too much, -- Cheers David