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

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Dec 05, 2025 at 10:34:48PM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/5/25 21:54, Kiryl Shutsemau wrote:
> > On Fri, Dec 05, 2025 at 09:44:30PM +0100, David Hildenbrand (Red Hat) wrote:
> > > On 12/5/25 21:33, Kiryl Shutsemau wrote:
> > > > On Fri, Dec 05, 2025 at 09:16:08PM +0100, David Hildenbrand (Red Hat) wrote:
> > > > > 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?
> > > > 
> > > > Sure. My understanding is that it is going to eliminate a need in
> > > > compound_head() completely. I don't see the conflict so far.
> > > 
> > > Right. All compound_head pointers will point at the allocated memdesc.
> > > 
> > > Would we still have to detect fake head pages though (at least for some
> > > transition period)?
> > 
> > If we need to detect if the memdesc is tail it should be as trivial as
> > comparing the given memdesc to the memdesc - 1. If they match, you are
> > looking at the tail.
> 
> How could you assume memdesc - 1 exists without performing other checks?

Map zero page in front of every discontinuous vmemmap region :P

-- 
  Kiryl Shutsemau / Kirill A. Shutemov