Re: [RFC PATCH 4/8] mm/vmalloc: Eliminate page table zigzag for huge vmalloc mappings

[Mike Rapoport <rppt@kernel.org>]

On Wed, Apr 08, 2026 at 10:51:11AM +0800, Barry Song (Xiaomi) wrote:
> For vmalloc() allocations with VM_ALLOW_HUGE_VMAP, we no longer
> need to iterate over pages one by one, which would otherwise lead to
> zigzag page table mappings.
> 
> The code is now unified with the PAGE_SHIFT case by simply
> calling vmap_small_pages_range_noflush().
> 
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/vmalloc.c | 22 ++++------------------
>  1 file changed, 4 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 5bf072297536..eba436386929 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -689,27 +689,13 @@ static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
>  int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>  		pgprot_t prot, struct page **pages, unsigned int page_shift)
>  {
> -	unsigned int i, nr = (end - addr) >> PAGE_SHIFT;
> -
>  	WARN_ON(page_shift < PAGE_SHIFT);
>  
> -	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> -			page_shift == PAGE_SHIFT)
> -		return vmap_small_pages_range_noflush(addr, end, prot, pages, PAGE_SHIFT);
> -
> -	for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> -		int err;
> -
> -		err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> -					page_to_phys(pages[i]), prot,
> -					page_shift);
> -		if (err)
> -			return err;
> +	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC))
> +		page_shift = PAGE_SHIFT;
>  
> -		addr += 1UL << page_shift;
> -	}
> -
> -	return 0;
> +	return vmap_small_pages_range_noflush(addr, end, prot, pages,
> +			min(page_shift, PMD_SHIFT));

Wouldn't vmap_range_noflush() already "do the right thing" even without
changes to vmap_small_pages_range_noflush()?

>  }
>  
>  int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
> -- 
> 2.39.3 (Apple Git-146)
> 

-- 
Sincerely yours,
Mike.