linux-kernel - Re: [PATCHv6 06/17] LoongArch/mm: Align vmemmap to maximal folio size

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Message-ID: <aYSgQknBNs5r-Tf5@thinkstation>
Date: Thu, 5 Feb 2026 13:52:30 +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 06/17] LoongArch/mm: Align vmemmap to maximal folio size

On Wed, Feb 04, 2026 at 05:56:45PM +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.
> > 
> > Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> > ---
> >   arch/loongarch/include/asm/pgtable.h | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
> > index c33b3bcb733e..f9416acb9156 100644
> > --- a/arch/loongarch/include/asm/pgtable.h
> > +++ b/arch/loongarch/include/asm/pgtable.h
> > @@ -113,7 +113,8 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
> >   	 min(PTRS_PER_PGD * PTRS_PER_PUD * PTRS_PER_PMD * PTRS_PER_PTE * PAGE_SIZE, (1UL << cpu_vabits) / 2) - PMD_SIZE - VMEMMAP_SIZE - KFENCE_AREA_SIZE)
> >   #endif
> > -#define vmemmap		((struct page *)((VMALLOC_END + PMD_SIZE) & PMD_MASK))
> > +#define VMEMMAP_ALIGN	max(PMD_SIZE, MAX_FOLIO_NR_PAGES * sizeof(struct page))
> > +#define vmemmap		((struct page *)(ALIGN(VMALLOC_END, VMEMMAP_ALIGN)))
> 
> 
> Same comment, the "MAX_FOLIO_NR_PAGES * sizeof(struct page)" is just black magic here
> and the description of the situation is wrong.
> 
> Maybe you want to pull the magic "MAX_FOLIO_NR_PAGES * sizeof(struct page)" into the core and call it
> 
> #define MAX_FOLIO_VMEMMAP_ALIGN	(MAX_FOLIO_NR_PAGES * sizeof(struct page))
> 
> But then special case it base on (a) HVO being configured in an (b) HVO being possible
> 
> #ifdef HUGETLB_PAGE_OPTIMIZE_VMEMMAP && is_power_of_2(sizeof(struct page)

This would require some kind of asm-offsets.c/bounds.c magic to pull the
struct page size condition to the preprocessor level.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov