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

["David Hildenbrand (arm)" <david@kernel.org>]

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@kernel.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