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Mon, 22 Dec 2025 09:52:20 -0500 (EST) Date: Mon, 22 Dec 2025 14:52:19 +0000 From: Kiryl Shutsemau To: "David Hildenbrand (Red Hat)" , Wilcox Cc: Muchun Song , 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 Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment Message-ID: References: <20251218150949.721480-1-kas@kernel.org> <20251218150949.721480-3-kas@kernel.org> <430d554c-840f-4813-b715-5191d74571a0@linux.dev> <4ctnfkw5zqneume6px2wqgdgusfdricr4uuonv7bfjheknrt2g@fdb5ceiiar4q> <3b758468-9985-49b8-948a-e5837decf52d@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3b758468-9985-49b8-948a-e5837decf52d@kernel.org> X-Stat-Signature: xkc41j4wgh7jdb7k3q6bmmhddatehtph X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 68E431C0005 X-Rspam-User: X-HE-Tag: 1766415143-761597 X-HE-Meta: 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 0HcLAo1E brWVwZc8jTOKb0HlutTOhy+Jn34BZATKCB8xuXkjNVrGOKDpKDP0h+9tdaTm7/h1MPI3NF/AkFnKIg71/qXWXJJKaqJtuJeLO1/36CRooaTkr+a+3GYF91GeOo/lAhPDJxWyvh8j1gapcdlxk6esvM5D1EbqxX2XYDbEHoONWxIce8IyBXIYEAcQTgwe/YmEyvWlWdE2RkAH5Z7mpbiA95XXBgIAus0XDs/9hAkW681SCreQOpAX7Ul5d743WTapMn2GfJnMGRdNCpV8= 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 Mon, Dec 22, 2025 at 03:18:29PM +0100, David Hildenbrand (Red Hat) wrote: > 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? It depends on memory model and whether the arch has KASLR for memmap. > 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, Willy, what is timeline here? -- Kiryl Shutsemau / Kirill A. Shutemov