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

Muhammad Usama Anjum
2026-03-19T12:59:08Z

On 19/03/2026 12:22 pm, Ryan Roberts wrote:
> 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.
I've looked at this again and simplified based on the fact tha
__GFP_SKIP_KASAN is zero in other than hw-tag modes.

>
>> /*
>> * 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.
Then it would be confusing to set kasan_flags to KASAN_VMALLOC_PROT_NORMAL and
not use it later. I've found a good of doing it this way.

Thanks,
Usama

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