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Thu, 5 Feb 2026 08:44:03 -0500 (EST) Date: Thu, 5 Feb 2026 13:43:58 +0000 From: Kiryl Shutsemau To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org Subject: Re: [PATCHv6 06/17] LoongArch/mm: Align vmemmap to maximal folio size Message-ID: References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-7-kas@kernel.org> <2ce0e684-de54-43ec-be7d-c58bbffb3f4e@kernel.org> <062900fa-6419-4748-81d1-9128ce6c46d0@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <062900fa-6419-4748-81d1-9128ce6c46d0@kernel.org> On Thu, Feb 05, 2026 at 01:56:36PM +0100, David Hildenbrand (Arm) wrote: > On 2/4/26 17:56, 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 > > > --- > > >   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. > > > > Thinking about this ... > > the vmemmap start is always struct-page-aligned. Otherwise we'd be in > trouble already. > > Isn't it then sufficient to just align the start to MAX_FOLIO_NR_PAGES? > > Let's assume sizeof(struct page) == 64 and MAX_FOLIO_NR_PAGES = 512 for > simplicity. > > vmemmap start would be multiples of 512 (0x0010000000). > > 512, 1024, 1536, 2048 ... > > Assume we have an 256-pages folio at 1536+256 = 0x111000000 s/0x/0b/, but okay. > Assume we have the last page of that folio (0x011111111111), we would just > get to the start of that folio by AND-ing with ~(256-1). > > Which case am I ignoring? IIUC, you are ignoring the actual size of struct page. It is not 1 byte :P The last page of this 256-page folio is at 1536+256 + (64 * 255) which is 0b100011011000000. There's no mask that you can AND that gets you to 0b11100000000. -- Kiryl Shutsemau / Kirill A. Shutemov