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From: Muchun Song Mime-Version: 1.0 (1.0) Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment Date: Mon, 22 Dec 2025 22:55:16 +0800 Message-Id: References: <3b758468-9985-49b8-948a-e5837decf52d@kernel.org> Cc: Kiryl Shutsemau , Matthew Wilcox , 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 In-Reply-To: <3b758468-9985-49b8-948a-e5837decf52d@kernel.org> To: David Hildenbrand X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: B75A940009 X-Stat-Signature: nukkq1588jfzs4x84rc6t45pz8fk4its X-Rspam-User: X-HE-Tag: 1766415365-282829 X-HE-Meta: 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 zUBw/KFa k7fsazwjF1P7C2h5g0aJZroIXuU9y7U68nuHFTpA3zy1svQ3O/9myCL912P6n6GJyklvRnoxO1pTfTZzaohJWzF4o69dBdpoeAgSuL8OM3X+xQhCxvz4r4YEeVuEv52JiQ+iTezOpNef4xyc25THEBg97wyWAEqCjt5bx1Ik261CTs+VY2XLkAez82vs0q07aWFdQo2BFu1OXk2xXGmPXiM7HB0fHrbQmn+i5 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 Dec 22, 2025, at 22:18, David Hildenbrand (Red Hat) w= rote: >=20 > =EF=BB=BFOn 12/22/25 15:02, Kiryl Shutsemau wrote: >>> On Mon, Dec 22, 2025 at 04:34:40PM +0800, Muchun Song wrote: >>>=20 >>>=20 >>> 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. >>>>=20 >>>> Add a warning if it is not. >>>>=20 >>>> 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. >>>=20 >>> 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=E2=80=99m 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=E2=80=99t >>> think a WARNING is a good choice. >> We can upgrade it BUG_ON(), but I want to understand your logic here >> first. >=20 > Definitely no BUG_ON(). I would assume this is something we would find ear= ly during testing, so even a VM_WARN_ON_ONCE() should be good enough? >=20 > This smells like a possible problem, though, as soon as some architecture w= ants to increase the folio size. What would be the expected step to ensure t= he alignment is done properly? >=20 > But OTOH, as I raised Willy's work will make all of that here obsolete eit= her way, so maybe not worth worrying about that case too much, Hi David, I hope you're doing well. I must admit I have limited knowledge of Willy's w= ork, and I was wondering if you might be kind enough to share any publicly a= vailable links where I could learn more about the future direction of this p= roject. I would be truly grateful for your guidance. Thank you very much in advance. Best regards, >=20 > -- > Cheers >=20 > David