linux-kernel - Re: [PATCH 1/2] mm/debug_vm_pgtable: Use set

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Message-ID: <12ac22e3-d943-431e-a3da-15987fbb458e@arm.com>
Date: Thu, 11 Dec 2025 09:36:27 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Samuel Holland <samuel.holland@...ive.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 David Hildenbrand <david@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Jann Horn <jannh@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Anshuman Khandual <anshuman.khandual@....com>, Gavin Shan
 <gshan@...hat.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH 1/2] mm/debug_vm_pgtable: Use set_pXd() to write page
 tables

On 11/12/2025 08:11, Samuel Holland wrote:
> Generic code must always use the architecture-provided helper function
> to write page tables.
> 
> Fixes: a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")
> Fixes: c0fe07b0aa72 ("mm/debug_vm_pgtable: use struct pgtable_debug_args in PMD modifying tests")
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> 
>  mm/debug_vm_pgtable.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index ae9b9310d96fd..8c7996d6c1f2a 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -445,7 +445,7 @@ static void __init pmd_huge_tests(struct pgtable_debug_args *args)
>  	 * X86 defined pmd_set_huge() verifies that the given
>  	 * PMD is not a populated non-leaf entry.
>  	 */
> -	WRITE_ONCE(*args->pmdp, __pmd(0));
> +	set_pmd(args->pmdp, __pmd(0));

My guess is that this should actually be:

	pmd_clear(args->pmdp);

As the "clear" value may not be zero on some arches (see um) or there may be
more to do on other arches (see arm32's 2 level pgtable).

>  	WARN_ON(!pmd_set_huge(args->pmdp, __pfn_to_phys(args->fixed_pmd_pfn), args->page_prot));
>  	WARN_ON(!pmd_clear_huge(args->pmdp));
>  	pmd = pmdp_get(args->pmdp);
> @@ -465,7 +465,7 @@ static void __init pud_huge_tests(struct pgtable_debug_args *args)
>  	 * X86 defined pud_set_huge() verifies that the given
>  	 * PUD is not a populated non-leaf entry.
>  	 */
> -	WRITE_ONCE(*args->pudp, __pud(0));
> +	set_pud(args->pudp, __pud(0));

Likewise, I think this should be:

	pud_clear(args->pudp);

Thanks,
Ryan

>  	WARN_ON(!pud_set_huge(args->pudp, __pfn_to_phys(args->fixed_pud_pfn), args->page_prot));
>  	WARN_ON(!pud_clear_huge(args->pudp));
>  	pud = pudp_get(args->pudp);