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Mon, 16 Feb 2026 11:20:06 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 6UJ7HGb9kmnSbAAAD6G6ig (envelope-from ); Mon, 16 Feb 2026 11:20:06 +0000 Message-ID: Date: Mon, 16 Feb 2026 12:20:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 08/17] mm: Make page_zonenum() use head page Content-Language: en-US To: "David Hildenbrand (Arm)" , Matthew Wilcox , Kiryl Shutsemau Cc: Andrew Morton , Muchun Song , Usama Arif , Frank van der Linden , Oscar Salvador , Mike Rapoport , 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: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: B293340005 X-Stat-Signature: jezzj1bebf8j1o1yft8xm4gk6bnd473i X-Rspam-User: X-HE-Tag: 1771240808-423 X-HE-Meta: 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 c0MnDuBd K0GZzR7mTcP/XUYRt3r5vU54aUJdOqWTFR9eo0Caklz/ouNh/9GAv1aMnJg2WQ7tRE6Hd5fKwGGzyIVxixDgR+Yuq2tnxkgGppxsQkDI8VObvhdMlxRYJd+aonlM23hS2vVGXQO+iezXgBA/A8vgqKZcmUhKIHGOOJe6PtpitQfarhACeh+qr5gf00S08or7clHDAI1t/A24II9k1OhKk5bkapPsKX6KKwR+8y6uduQ2I5SR85fcC+elKN4TUh1EwsaGa8GluIXhpaFBBv3iDJ8xQ6++hNiBA1uSyNsiOBBOX2Mks/5vY+q3dsxcSDk0uIUkgC1FS3iVN9T4= 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 10:06, David Hildenbrand (Arm) wrote: > 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. I would be surprised if things didn't break horribly if we allowed crossing zones in a single folio. I'd rather not allow it. (And I still don't like how this patch solves the issue) >> >> 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. >