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b=FYvIch1U38DwPufVNrBLBAbn0caX9A0u2fc1XTx+8xS4GM6lbSoz0aK6WztCOQhgu Vx0h8p4NW70I4RYvdbzNN3WeCbH2veDYorXEU+XuRnOO2bBGmDyuj0HdvJ0UO9NapA WMmllVlT3Sps3rZOVIgM21JGVU3rPlmkTd/PrNsaqsDBCNcKojNsZ8ARxqvwtAxRsI TgoqrGz+UUyTZMy5qOlBHvr/5o0DCg0PWIfE7yqHUeBhz0YnA2tbX9c9NMxbQZ18nI H0ZFyO/ROTLHJ/Eh3O1IjbDs9b2IQPw4FXOY0/N4Vsc9RofntPgvkSjl5odPTVBJV7 RbqdaQo9YbeBw== Message-ID: Date: Mon, 16 Feb 2026 10:06:57 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 08/17] mm: Make page_zonenum() use head page To: Matthew Wilcox , Kiryl Shutsemau Cc: Andrew Morton , Muchun Song , Usama Arif , Frank van der Linden , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-9-kas@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-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 3CD7CA0009 X-Stat-Signature: qw91x9xxw4ekgyrzr3k6bzb8qdpdqrdz X-Rspam-User: X-HE-Tag: 1771232828-830414 X-HE-Meta: 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 UYWWJf7M TnZMk5VJw09YTpRlmYgVJnWaIqOo/LWMtzUozmJIt/qDGhdTvxzks8ROb+fozmir/eEyn1+T2lWeicN3Xezmm0m4Uqh/DwDdsXfS6gkKksXKJBNvmkGmU7cdU7+HfcRrj6wa5Eq9Fnia37JgcS4v5SIxpmKkDP8flpjhydXJwpyfxFC+qXCBc5jQnRBJoXmuquaWxRCwL7gZq4z16amtm41puf1gcTj7rpMUq4UMcaSFPIs5MDiExLA//31IEHKOjjO5RI7U4evxuvi8cGOywT+HhGZGtCHUZ2G7hQzcr5sfdHyPRU4Gt4maf3g== 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 2/16/26 00:13, Matthew Wilcox wrote: > On Mon, Feb 02, 2026 at 03:56:24PM +0000, Kiryl Shutsemau wrote: >> With the upcoming changes to HVO, a single page of tail struct pages >> will be shared across all huge pages of the same order on a node. Since >> huge pages on the same node may belong to different zones, the zone >> information stored in shared tail page flags would be incorrect. >> >> Always fetch zone information from the head page, which has unique and >> correct zone flags for each compound page. > > You're right that different pages in the same folio can have different > zone number. But does it matter ... or to put it another way, why is > returning the zone number of the head page the correct way to resolve > this? How can a folio cross zones? Runtime allocated hugetlb folios from the CMA/buddy (alloc_contig_range) definitely fall into a single zone. So is it about ones allocated early during boot, where, by chance, we manage to cross ZONE_NORMAL + ZONE_MOVABLE etc? I thought that it's also not allowed there, and I wonder whether we should disallow it if it's possible. > > Arguably, the caller is asking for the zone number of _this page_, and > does not care about the zone number of the head page. It would be good > to have a short discussion of this in the commit message (but probably > not worth putting this in a comment). Agreed, in particular, if there would be a functional change. So far I assumed there would be no such change. Things like shrink_zone_span() really need to know the zone of that page, not the one of the head; unless both fall into the same zone. -- Cheers, David