Re: [PATCH v1 1/4] kasan: Avoid sleepable page allocation from atomic context

["Nicholas Piggin" <npiggin@gmail.com>]

On Tue Apr 8, 2025 at 1:11 AM AEST, 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.

It's a bit unfortunate to make this use atomic allocs for
archs that don't need it.

Could you make it depend on __HAVE_ARCH_ENTER_LAZY_MMU_MODE
or is that overkill?

I wanted to remove ppc64's per-CPU page array and replace it
with on stack or dynaimc alloc array in the thread... but
cost/benefit of working on ppc64 hash MMU code is not
high :(

Fix itself for ppc64's requirement at least looks right to me
so for that,

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

>
> 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;
>