linux-kernel - Re: [PATCHv2 06/14] mm: Rework compound_head() for power-of-2 sizeof(struct page)

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Message-Id: <34BED0AB-22AF-4107-84FB-543D11BCA136@linux.dev>
Date: Mon, 22 Dec 2025 15:57:40 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 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>,
 kernel-team@...a.com,
 linux-mm@...ck.org,
 linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCHv2 06/14] mm: Rework compound_head() for power-of-2
 sizeof(struct page)



> On Dec 18, 2025, at 23:09, Kiryl Shutsemau <kas@...nel.org> wrote:
> 
> For tail pages, the kernel uses the 'compound_info' field to get to the
> head page. The bit 0 of the field indicates whether the page is a
> tail page, and if set, the remaining bits represent a pointer to the
> head page.
> 
> For cases when size of struct page is power-of-2, change the encoding of
> compound_info to store a mask that can be applied to the virtual address
> of the tail page in order to access the head page. It is possible
> because struct page of the head page is naturally aligned with regards
> to order of the page.
> 
> The significant impact of this modification is that all tail pages of
> the same order will now have identical 'compound_info', regardless of
> the compound page they are associated with. This paves the way for
> eliminating fake heads.
> 
> The HugeTLB Vmemmap Optimization (HVO) creates fake heads and it is only
> applied when the sizeof(struct page) is power-of-2. Having identical
> tail pages allows the same page to be mapped into the vmemmap of all
> pages, maintaining memory savings without fake heads.
> 
> If sizeof(struct page) is not power-of-2, there is no functional
> changes.
> 

Forgot to mention, I believe I stated in the previous version that this
mechanism only applies when CONFIG_SPARSEMEM_VMEMMAP is configured.
Therefore, you need to wrap the entire mechanism within CONFIG_SPARSEMEM_VMEMMAP.
For other configurations, it's difficult to guarantee alignment to a very
large size (for example, in the case of CONFIG_SPARSEMEM && !CONFIG_SPARSEMEM_VMEMMAP,
vmemmap allocation uses kvmalloc, which only guarantees PAGE_SIZE alignment
for the returned address).