Re: [PATCH v2 1/3] vmalloc: add __GFP_SKIP_KASAN support

[David Hildenbrand <david.hildenbrand@arm.com>]

On 3/24/26 14:26, 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>
> ---
> Changes since v1:
> - Simplify skip conditions based on the fact that __GFP_SKIP_KASAN
>   is zero in non-hw-tags mode.
> - Add __GFP_SKIP_KASAN to GFP_VMALLOC_SUPPORTED list of flags
> ---
>  mm/vmalloc.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c607307c657a6..69ae205effb46 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3939,7 +3939,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  				__GFP_NOFAIL | __GFP_ZERO |\
>  				__GFP_NORETRY | __GFP_RETRY_MAYFAIL |\
>  				GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\
> -				GFP_USER | __GFP_NOLOCKDEP)
> +				GFP_USER | __GFP_NOLOCKDEP | __GFP_SKIP_KASAN)
>  
>  static gfp_t vmalloc_fix_flags(gfp_t flags)
>  {
> @@ -3980,6 +3980,8 @@ static gfp_t vmalloc_fix_flags(gfp_t flags)
>   *
>   * %__GFP_NOWARN can be used to suppress failure messages.
>   *
> + * %__GFP_SKIP_KASAN can be used to skip poisoning
> + *
>   * Can not be called from interrupt nor NMI contexts.
>   * Return: the address of the area or %NULL on failure
>   */
> @@ -4041,7 +4043,9 @@ 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 = gfp_mask & __GFP_SKIP_KASAN;
> +
> +		if (kasan_hw_tags_enabled() && !skip_kasan) {

This code gets ever more ugly. :)

After I spotted the horrible ___GFP_SKIP_ZERO that shouldn't even exist,
I thought about teaching vmalloc.c to use a sub-allocator interface to
the buddy instead, where we would essentially say "leave zeroing and
KASAN to the sub-allocator": vmalloc.

Then, we'd get rid of ___GFP_SKIP_ZERO and just use __GFP_SKIP_KASAN to
decide ourselves here what to do with KASAN.

I tried to implement that, but that SW KASAN / !KASAN handling messes
with my brain. :)

In particular, the order for HW KASAN is currently:

a) Allocate pages *and map them*.

b) Zero the pages

That means that we have temporarily unzeroed pages mapped there. I don't
know if that's problematic, but it's one of the differences to SW KASAN
/ ! KASAN handling here.

-- 
Cheers,

David