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

David Hildenbrand (Red Hat)
2025-12-05T20:44:38Z

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