Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap optimization

David Hildenbrand (Red Hat)
2025-12-05T20:16:16Z

On 12/5/25 20:43, Kiryl Shutsemau wrote:
> This series removes "fake head pages" from the HugeTLB vmemmap
> optimization (HVO) by changing how tail pages encode their relationship
> to the head page.
>
> It simplifies compound_head() and page_ref_add_unless(). Both are in the
> hot path.
>
> Background
> ==========
>
> HVO reduces memory overhead by freeing vmemmap pages for HugeTLB pages
> and remapping the freed virtual addresses to a single physical page.
> Previously, all tail page vmemmap entries were remapped to the first
> vmemmap page (containing the head struct page), creating "fake heads" -
> tail pages that appear to have PG_head set when accessed through the
> deduplicated vmemmap.
>
> This required special handling in compound_head() to detect and work
> around fake heads, adding complexity and overhead to a very hot path.
>
> New Approach
> ============
>
> For architectures/configs where sizeof(struct page) is a power of 2 (the
> common case), this series changes how position of the head page is encoded
> in the tail pages.
>
> Instead of storing a pointer to the head page, the ->compound_info
> (renamed from ->compound_head) now stores a mask.

(we're in the merge window)

That doesn't seem to be suitable for the memdesc plans, where we want
all tail pages do directly point at the allocated memdesc (e.g., struct
folio), no?

@Willy what's your take?

--
Cheers

David