linux-kernel - Re: [PATCHv6 08/17] mm: Make page

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Message-ID: <e5831f0b-88c5-4a20-82b3-6e5952388132@kernel.org>
Date: Thu, 5 Feb 2026 14:10:40 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Matthew Wilcox <willy@...radead.org>,
 Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com>
Cc: Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>,
 Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, kernel-team@...a.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-riscv@...ts.infradead.org
Subject: Re: [PATCHv6 08/17] mm: Make page_zonenum() use head page

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 <kas@...nel.org>
> Acked-by: Zi Yan <ziy@...dia.com>
> ---
>   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).

-- 
Cheers,

David