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

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Message-ID: <e1c50f7e-cec4-4676-b5d6-f6446cd64ba5@kernel.org>
Date: Fri, 5 Dec 2025 21:16:08 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Matthew Wilcox <willy@...radead.org>
Cc: Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>,
 Usama Arif <usamaarif642@...il.com>, kernel-team@...a.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap
 optimization

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