Re: [PATCHv6 08/17] mm: Make page_zonenum() use head page

[Kiryl Shutsemau <kas@kernel.org>]

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 <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).

I will add compound_info_has_mask() check.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov