Re: [PATCH 4/6] mm/vmalloc: make set_area_direct_map HUGE_VMAP friendly

["Brendan Jackman" <brendan.jackman@linux.dev>]

On Sun Aug 16, 2026 at 12:59 PM CEST, Mike Rapoport (Microsoft) wrote:
> set_area_direct_map() always updates direct map alias permissions in
> single page increments.
>
> For HUGE_VMAP areas it's suboptimal. Not only the loop in
> set_area_direct_map() needlessly has more iterations (e.g times 512 on
> x86), but it also causes fragmentation of the direct map that could be
> avoided for the HUGE_VMAP areas populated with large pages.
>
> All pages in an area are always of the same order: either same-order
> large pages when VM_ALLOW_HUGE_VMAP is set and all huge pages were
> successfully allocated, or order-0 page when VM_ALLOW_HUGE_VMAP is
> cleared or when huge pages allocation fails and fallback path is taken.
>
> Instead of updating the direct map permissions for every order-0 page in
> an area, use the area's page_order as the loop increment and update the
> large pages in one call to set_direct_map_{invalid,default}_noflush().
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  mm/vmalloc.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index fc7993db4152..11170d1ee5be 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3361,12 +3361,15 @@ static inline void set_area_direct_map(const struct vm_struct *area,
>  				       int (*set_direct_map)(struct page *page,
>  							     unsigned int nr))
>  {
> -	unsigned long i;
> +	unsigned int nr = (1U << vm_area_page_order(area));
> +
> +	for (unsigned long i = 0; i < area->nr_pages; i += nr) {
> +		if (page_address(area->pages[i])) {
> +			int err = set_direct_map(area->pages[i], nr);
>  
> -	/* HUGE_VMALLOC passes small pages to set_direct_map */
> -	for (i = 0; i < area->nr_pages; i++)
> -		if (page_address(area->pages[i]))
> -			set_direct_map(area->pages[i], 1);
> +			WARN_ON_ONCE(err);

Nit: Maybe worth a comment on why this is expected to always succeed?
I.e. I think we are assuming the only failure mode is allocation but 
because we know vm_area_page_order() there shouldn't be any allocation?