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

[Vlastimil Babka]

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 <kas at kernel.org>
>> > Acked-by: Zi Yan <ziy at 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.

Not thrilled by this indeed. Would it be a problem to have the shared tail
pages per node+zone instead of just per node?