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.

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;
 
-- 
2.45.2

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.
>
> 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;
>  

Oh of course you can't make it GFP_KERNEL after the
patch to take ptl even for archs that don't use lazy
mmu.

Thanks,
Nick

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;
>