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@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.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);
 }
 
-- 
2.51.2

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@kernel.org>
> Acked-by: Zi Yan <ziy@nvidia.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

> On Feb 2, 2026, at 23:56, Kiryl Shutsemau <kas@kernel.org> 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@kernel.org>
> Acked-by: Zi Yan <ziy@nvidia.com>

Acked-by: Muchun Song <muchun.song@linux.dev>