linux-kernel - Re: [PATCHv6 05/17] riscv/mm: Align vmemmap to maximal folio size

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Message-ID: <aYSe0TAIzxJ9i1Wy@thinkstation>
Date: Thu, 5 Feb 2026 13:50:18 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: "David Hildenbrand (arm)" <david@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Muchun Song <muchun.song@...ux.dev>, 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 05/17] riscv/mm: Align vmemmap to maximal folio size

On Wed, Feb 04, 2026 at 05:50:23PM +0100, David Hildenbrand (arm) wrote:
> On 2/2/26 16:56, Kiryl Shutsemau wrote:
> > The upcoming change to the HugeTLB vmemmap optimization (HVO) requires
> > struct pages of the head page to be naturally aligned with regard to the
> > folio size.
> > 
> > Align vmemmap to MAX_FOLIO_NR_PAGES.
> 
> I think neither that statement nor the one in the patch description is
> correct?
> 
> "MAX_FOLIO_NR_PAGES * sizeof(struct page)" is neither the maximum folio size
> nor MAX_FOLIO_NR_PAGES.
> 
> It's the size of the memmap that a large folio could span at maximum.
> 
> 
> Assuming we have a 16 GiB folio, the calculation would give us
> 
> 	4194304 * sizeof(struct page)
> 
> Which could be something like (assuming 80 bytes)
> 
> 	335544320
> 
> -> not even a power of 2, weird? (for HVO you wouldn't care as HVO would be
> disabled, but that aliment is super weird?)
> 
> 
> Assuming 64 bytes, it would be a power of two (as 64 is a power of two).
> 
> 	268435456 (1<< 28)
> 
> 
> Which makes me wonder whether there is a way to avoid sizeof(struct page)
> here completely.

I don't think we can. See the other thread.

What about using roundup_pow_of_two(sizeof(struct page)) here.

> Or limit the alignment to the case where HVO is actually active and
> sizeof(struct page) makes any sense?

The annoying part of HVO is that it is unknown at compile-time if it
will be used. You can compile kernel with HVO that will no be activated
due to non-power-of-2 sizeof(struct page) because of a debug config option.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov