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

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Message-ID: <3e245617-81a5-4ea3-843f-b86261cf8599@gmail.com>
Date: Wed, 9 Apr 2025 16:10:58 +0200
From: Andrey Ryabinin <ryabinin.a.a@...il.com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Hugh Dickins <hughd@...gle.com>, Nicholas Piggin <npiggin@...il.com>,
 Guenter Roeck <linux@...ck-us.net>, Juergen Gross <jgross@...e.com>,
 Jeremy Fitzhardinge <jeremy@...p.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, kasan-dev@...glegroups.com, sparclinux@...r.kernel.org,
 xen-devel@...ts.xenproject.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] kasan: Avoid sleepable page allocation from atomic
 context



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@...r.kernel.org
> Fixes: 3c5c3cfb9ef4 ("kasan: support backing vmalloc space with real shadow memory")
> Signed-off-by: Alexander Gordeev <agordeev@...ux.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.