Re: [PATCHv7 12/18] mm/hugetlb: Remove fake head pages

[Vlastimil Babka (SUSE)]

On 2/27/26 20:42, Kiryl Shutsemau (Meta) wrote:
> From: Kiryl Shutsemau <kas@xxxxxxxxxx>
> 
> 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 zone
> per order in the vmemmap_tails[] array in struct zone. All huge pages of
> that order in the zone share this tail page, mapped read-only into their
> vmemmap. The head page remains unique per huge page.
> 
> Redefine MAX_FOLIO_ORDER using ilog2(). The define has to produce a
> compile-constant as it is used to specify vmemmap_tail array size.
> For some reason, compiler is not able to solve get_order() at
> compile-time, but ilog2() works.
> 
> Avoid PUD_ORDER to define MAX_FOLIO_ORDER as it adds dependency to
> <linux/pgtable.h> which generates hard-to-break include loop.
> 
> 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@xxxxxxxxxx>

LGTM, glad you found a way to do the per-zone tails and not pessimize
page_zonenum().

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>