Re: [PATCH 1/3] vmalloc: add __GFP_SKIP_KASAN support

Ryan Roberts
2026-03-19T12:22:30Z

On 19/03/2026 11:49, Muhammad Usama Anjum wrote:
> For allocations that will be accessed only with match-all pointers
> (e.g., kernel stacks), setting tags is wasted work. If the caller
> already set __GFP_SKIP_KASAN, don’t skip zeroing the pages and
> don’t set KASAN_VMALLOC_PROT_NORMAL so kasan_unpoison_vmalloc()
> returns early without tagging.
>
> Before this patch, __GFP_SKIP_KASAN wasn't being used with vmalloc
> APIs. So it wasn't being checked. Now its being checked and acted
> upon. Other KASAN modes are unchanged because __GFP_SKIP_KASAN isn't
> defined there.
>
> This is a preparatory patch for optimizing kernel stack allocations.
>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> ---
> mm/vmalloc.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c607307c657a6..1baa602a0b9bb 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4041,7 +4041,10 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
> * kasan_unpoison_vmalloc().
> */
> if (pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) {
> - if (kasan_hw_tags_enabled()) {
> + bool skip_kasan = kasan_hw_tags_enabled() &&
> + (gfp_mask & __GFP_SKIP_KASAN);
> +
> + if (kasan_hw_tags_enabled() && !skip_kasan) {

It's unfortunate that kasan_hw_tags_enabled() is involved twice in this expression.

> /*
> * Modify protection bits to allow tagging.
> * This must be done before mapping.
> @@ -4057,7 +4060,8 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
> }
>
> /* Take note that the mapping is PAGE_KERNEL. */
> - kasan_flags |= KASAN_VMALLOC_PROT_NORMAL;
> + if (!skip_kasan)
> + kasan_flags |= KASAN_VMALLOC_PROT_NORMAL;

I wonder if it would be clearer to just not call kasan_unpoison_vmalloc() below
if the user passed in __GFP_SKIP_KASAN? It's really just an implementation
detail that kasan_unpoison_vmalloc() skips unpoisoning if
KASAN_VMALLOC_PROT_NORMAL is not provided.

Thanks,
Ryan

> }
>
> /* Allocate physical pages and map them into vmalloc space. */