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

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Message-ID: <45b8ab60-8721-414b-a54c-008269e28a90@kernel.org>
Date: Thu, 5 Feb 2026 14:57:24 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...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 2/5/26 14:52, Kiryl Shutsemau wrote:
> 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.
> 

Right.

I guess you could move that into the macro and let the compiler handle it.

-- 
Cheers,

David