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Mon, 23 Feb 2026 09:52:30 -0500 (EST) Date: Mon, 23 Feb 2026 14:52:25 +0000 From: Kiryl Shutsemau To: Vlastimil Babka Cc: "David Hildenbrand (Arm)" , Andrew Morton , Muchun Song , Matthew Wilcox , 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 Subject: Re: [PATCHv6 08/17] mm: Make page_zonenum() use head page Message-ID: References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-9-kas@kernel.org> <6afd7334-82e8-4f87-8ed3-6448e89328c2@suse.cz> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Feb 16, 2026 at 11:30:22AM +0000, Kiryl Shutsemau wrote: > On Tue, Feb 10, 2026 at 04:57:55PM +0100, Vlastimil Babka wrote: > > On 2/9/26 12:52, Kiryl Shutsemau wrote: > > > On Thu, Feb 05, 2026 at 02:10:40PM +0100, David Hildenbrand (Arm) wrote: > > >> On 2/2/26 16:56, 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. > > >> > > > >> > Signed-off-by: Kiryl Shutsemau > > >> > Acked-by: Zi Yan > > >> > --- > > >> > include/linux/mmzone.h | 1 + > > >> > 1 file changed, 1 insertion(+) > > >> > > > >> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > > >> > index be8ce40b5638..192143b5cdc0 100644 > > >> > --- a/include/linux/mmzone.h > > >> > +++ b/include/linux/mmzone.h > > >> > @@ -1219,6 +1219,7 @@ static inline enum zone_type memdesc_zonenum(memdesc_flags_t flags) > > >> > static inline enum zone_type page_zonenum(const struct page *page) > > >> > { > > >> > + page = compound_head(page); > > >> > return memdesc_zonenum(page->flags); > > >> > > >> We end up calling page_zonenum() without holding a reference. > > >> > > >> Given that _compound_head() does a READ_ONCE(), this should work even if we > > >> see concurrent page freeing etc. > > >> > > >> However, this change implies that we now perform a compound page lookup for > > >> every PageHighMem() [meh], page_zone() [quite some users in the buddy, > > >> including for pageblock access and page freeing]. > > >> > > >> That's a nasty compromise for making HVO better? :) > > >> > > >> We should likely limit that special casing to kernels that really rquire it > > >> (HVO). > > > > > > I will add compound_info_has_mask() check. > > > > Not thrilled by this indeed. Would it be a problem to have the shared tail > > pages per node+zone instead of just per node? > > I thought it would be overkill. It likely is going to be unused for most > nodes. But sure, move it to per-zone. I gave it a try, but stumbled on a problem. We need to know the zone in hugetlb_vmemmap_init_early(), but zones are not yet defined. hugetlb_vmemmap_init_early() is called from within sparse_init(), but span of zones is defined in free_area_init() after sparse_init(). Any ideas, how get past this? :/ -- Kiryl Shutsemau / Kirill A. Shutemov