Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages

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

On 1/15/26 15:45, Kiryl Shutsemau wrote:
> HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most
> vmemmap pages for huge pages and remapping the freed range to a single
> page containing the struct page metadata.
> 
> With the new mask-based compound_info encoding (for power-of-2 struct
> page sizes), all tail pages of the same order are now identical
> regardless of which compound page they belong to. This means the tail
> pages can be truly shared without fake heads.
> 
> Allocate a single page of initialized tail struct pages per NUMA node
> per order in the vmemmap_tails[] array in pglist_data. All huge pages
> of that order on the node share this tail page, mapped read-only into
> their vmemmap. The head page remains unique per huge page.
> 
> This eliminates fake heads while maintaining the same memory savings,
> and simplifies compound_head() by removing fake head detection.
> 
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> ---
>   include/linux/mmzone.h | 16 ++++++++++++++-
>   mm/hugetlb_vmemmap.c   | 44 ++++++++++++++++++++++++++++++++++++++++--
>   mm/sparse-vmemmap.c    | 44 ++++++++++++++++++++++++++++++++++--------
>   3 files changed, 93 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 322ed4c42cfc..2ee3eb610291 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -82,7 +82,11 @@
>    * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
>    * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
>    */
> -#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
> +#ifdef CONFIG_64BIT
> +#define MAX_FOLIO_ORDER		(34 - PAGE_SHIFT)
> +#else
> +#define MAX_FOLIO_ORDER		(30 - PAGE_SHIFT)
> +#endif

Where do these magic values stem from, and how do they related to the 
comment above that clearly spells out 16G vs. 1G ?


-- 
Cheers

David