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

["David Hildenbrand (Arm)" <david@kernel.org>]

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