Re: [PATCH v2 1/3] kasan: Avoid sleepable page allocation from atomic context

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

On 4/8/25 6:07 PM, Alexander Gordeev wrote:
> apply_to_page_range() enters lazy MMU mode and then invokes
> kasan_populate_vmalloc_pte() callback on each page table walk
> iteration. The lazy MMU mode may only be entered only under
> protection of the page table lock. However, the callback can
> go into sleep when trying to allocate a single page.
> 
> Change __get_free_page() allocation mode from GFP_KERNEL to
> GFP_ATOMIC to avoid scheduling out while in atomic context.
> 
> Cc: stable@vger.kernel.org
> Fixes: 3c5c3cfb9ef4 ("kasan: support backing vmalloc space with real shadow memory")
> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
> ---
>  mm/kasan/shadow.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index 88d1c9dcb507..edfa77959474 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -301,7 +301,7 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>  	if (likely(!pte_none(ptep_get(ptep))))
>  		return 0;
>  
> -	page = __get_free_page(GFP_KERNEL);
> +	page = __get_free_page(GFP_ATOMIC);
>  	if (!page)
>  		return -ENOMEM;
>  

I think a better way to fix this would be moving out allocation from atomic context. Allocate page prior
to apply_to_page_range() call and pass it down to kasan_populate_vmalloc_pte().

Whenever kasan_populate_vmalloc_pte() will require additional page we could bail out with -EAGAIN,
and allocate another one.