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

[David Hildenbrand (Arm)]

On 2/16/26 00:13, Matthew Wilcox wrote:
> On Mon, Feb 02, 2026 at 03:56:24PM +0000, 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.
>
> You're right that different pages in the same folio can have different
> zone number.  But does it matter ... or to put it another way, why is
> returning the zone number of the head page the correct way to resolve
> this?

How can a folio cross zones?

Runtime allocated hugetlb folios from the CMA/buddy (alloc_contig_range) 
definitely fall into a single zone.

So is it about ones allocated early during boot, where, by chance, we 
manage to cross ZONE_NORMAL + ZONE_MOVABLE etc?

I thought that it's also not allowed there, and I wonder whether we 
should disallow it if it's possible.

> Arguably, the caller is asking for the zone number of _this page_, and
> does not care about the zone number of the head page.  It would be good
> to have a short discussion of this in the commit message (but probably
> not worth putting this in a comment).

Agreed, in particular, if there would be a functional change. So far I 
assumed there would be no such change.

Things like shrink_zone_span() really need to know the zone of that 
page, not the one of the head; unless both fall into the same zone.

--
Cheers,

David