Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap optimization
From: David Hildenbrand (Red Hat)
Date: Fri Dec 05 2025 - 15:44:38 EST
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)?
I don't recall whether we'll really convert all memdesc users at once, or if some memdescs will co-exist with ordinary compound pages for a while.
--
Cheers
David