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

[Dev Jain <dev.jain@arm.com>]

On 22/04/26 6:51 pm, Ryan Roberts wrote:
> On 24/03/2026 13:26, 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
> 
> You mean skip *un*poisoning, I think? But you would only want this to apply to
> the actaul pages mapped by vmalloc. You wouldn't want to skip unpoisoning for
> any allocated meta data; I think that is currently possible since the gfp_flags
> that are passed into __vmalloc_node_range_noprof() are passed down to
> __get_vm_area_node() unmdified. You probably want to explicitly ensure
> __GFP_SKIP_KASAN is clear for that internal call?
> 
>> + *
>>   * 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) {
>>  			/*
>>  			 * Modify protection bits to allow tagging.
>>  			 * This must be done before mapping.
>> @@ -4057,7 +4061,8 @@ 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;
> 
> It's pretty ugly to use the absence of this flag to rely on
> kasan_unpoison_vmalloc() not unpoisoning. Perhaps it is preferable to just not
> call kasan_unpoison_vmalloc() for the skip_kasan case?
> 
>>  	}
>>  
>>  	/* Allocate physical pages and map them into vmalloc space. */
> 
> Perhaps something like this would work:
> 
> ---8<---
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c31a8615a8328..c340db141df57 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3979,6 +3979,8 @@ static gfp_t vmalloc_fix_flags(gfp_t flags)
>   * under moderate memory pressure.
>   *
>   * %__GFP_NOWARN can be used to suppress failure messages.
> +
> + * %__GFP_SKIP_KASAN skip unpoisoning of mapped pages (when prot=PAGE_KERNEL).
>   *
>   * Can not be called from interrupt nor NMI contexts.
>   * Return: the address of the area or %NULL on failure
> @@ -3993,6 +3995,9 @@ void *__vmalloc_node_range_noprof(unsigned long size,
> unsigned long align,
>  	kasan_vmalloc_flags_t kasan_flags = KASAN_VMALLOC_NONE;
>  	unsigned long original_align = align;
>  	unsigned int shift = PAGE_SHIFT;
> +	bool skip_kasan = gfp_mask & __GFP_SKIP_KASAN;
> +
> +	gfp_mask &= ~__GFP_SKIP_KASAN;

Okay so this is so that metadata allocation can keep using normal
page allocator side unpoisoning.

>   	if (WARN_ON_ONCE(!size))
>  		return NULL;
> @@ -4041,7 +4046,7 @@ 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()) {
> +		if (kasan_hw_tags_enabled() && !skip_kasan) {

Why do we want to elide GFP_SKIP_ZERO (set below) in this case?

>  			/*
>  			 * Modify protection bits to allow tagging.
>  			 * This must be done before mapping.
> @@ -4054,6 +4059,12 @@ void *__vmalloc_node_range_noprof(unsigned long size,
> unsigned long align,
>  			 * poisoned and zeroed by kasan_unpoison_vmalloc().
>  			 */
>  			gfp_mask |= __GFP_SKIP_KASAN | __GFP_SKIP_ZERO;
> +		} else if (skip_kasan) {
> +			/*
> +			 * Skip page_alloc unpoisoning physical pages backing
> +			 * VM_ALLOC mapping, as requested by caller.
> +			 */
> +			gfp_mask |= __GFP_SKIP_KASAN;
>  		}
>   		/* Take note that the mapping is PAGE_KERNEL. */
> @@ -4078,7 +4089,8 @@ void *__vmalloc_node_range_noprof(unsigned long size,
> unsigned long align,
>  	    (gfp_mask & __GFP_SKIP_ZERO))
>  		kasan_flags |= KASAN_VMALLOC_INIT;
>  	/* KASAN_VMALLOC_PROT_NORMAL already set if required. */
> -	area->addr = kasan_unpoison_vmalloc(area->addr, size, kasan_flags);
> +	if (!skip_kasan)
> +		area->addr = kasan_unpoison_vmalloc(area->addr, size, kasan_flags);

I really think we should do some decoupling here - GFP_SKIP_KASAN means,
"skip KASAN when going through page allocator". Now we reuse this flag
to skip vmalloc unpoisoning.

Some code path using GFP_SKIP_KASAN (which is highly likely given that
GFP_HIGHUSER_MOVABLE has this) and also using vmalloc() will unintentionally
also skip vmalloc unpoisoning.

I think we are doing patch 1 because of patch 2 - so in patch 2, perhaps
instead of calling __vmalloc_node we can call __vmalloc_node_range_noprof and
shift this "skip vmalloc unpoisoning" functionality into vmalloc flags instead?
Perhaps this won't work for the nommu case (__vmalloc_node has two definitions),
just a line of thought.


>   	/*
>  	 * In this function, newly allocated vm_struct has VM_UNINITIALIZED
> 
> ---8<---
> 
> Thanks,
> Ryan
> 
>