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Mon, 16 Feb 2026 06:30:20 -0500 (EST) Date: Mon, 16 Feb 2026 11:30:13 +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: <6afd7334-82e8-4f87-8ed3-6448e89328c2@suse.cz> 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. -- Kiryl Shutsemau / Kirill A. Shutemov