linux-kernel - Re: [PATCHv6 11/17] mm/hugetlb: Remove fake head pages

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Message-Id: <1FFEED62-343D-433C-BB81-EB646D066AC9@linux.dev>
Date: Tue, 3 Feb 2026 17:50:34 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Matthew Wilcox <willy@...radead.org>,
 Usama Arif <usamaarif642@...il.com>,
 Frank van der Linden <fvdl@...gle.com>,
 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>,
 Huacai Chen <chenhuacai@...nel.org>,
 WANG Xuerui <kernel@...0n.name>,
 Palmer Dabbelt <palmer@...belt.com>,
 Paul Walmsley <paul.walmsley@...ive.com>,
 Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>,
 kernel-team@...a.com,
 linux-mm@...ck.org,
 linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org,
 loongarch@...ts.linux.dev,
 linux-riscv@...ts.infradead.org
Subject: Re: [PATCHv6 11/17] mm/hugetlb: Remove fake head pages



> On Feb 2, 2026, at 23:56, Kiryl Shutsemau <kas@...nel.org> wrote:
> 
> HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most
> vmemmap pages for huge pages and remapping the freed range to a single
> page containing the struct page metadata.
> 
> With the new mask-based compound_info encoding (for power-of-2 struct
> page sizes), all tail pages of the same order are now identical
> regardless of which compound page they belong to. This means the tail
> pages can be truly shared without fake heads.
> 
> Allocate a single page of initialized tail struct pages per NUMA node
> per order in the vmemmap_tails[] array in pglist_data. All huge pages of
> that order on the node share this tail page, mapped read-only into their
> vmemmap. The head page remains unique per huge page.
> 
> Redefine MAX_FOLIO_ORDER using ilog2(). The define has to produce a
> compile-constant as it is used to specify vmemmap_tail array size.
> For some reason, compiler is not able to solve get_order() at
> compile-time, but ilog2() works.
> 
> Avoid PUD_ORDER to define MAX_FOLIO_ORDER as it adds dependency to
> <linux/pgtable.h> which generates hard-to-break include loop.
> 
> This eliminates fake heads while maintaining the same memory savings,
> and simplifies compound_head() by removing fake head detection.
> 
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>

Reviewed-by: Muchun Song <muchun.song@...ux.dev>