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

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Message-ID: <2ce0e684-de54-43ec-be7d-c58bbffb3f4e@kernel.org>
Date: Wed, 4 Feb 2026 17:56:45 +0100
From: "David Hildenbrand (arm)" <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>,
 Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com>
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>,
 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 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)
/* A very helpful comment explaining the situation. */
#define MAX_FOLIO_VMEMMAP_ALIGN	(MAX_FOLIO_NR_PAGES * sizeof(struct page))
#else
#define MAX_FOLIO_VMEMMAP_ALIGN	0
#endif

Something like that.

-- 
Cheers,

David