Re: [PATCH] mm/debug_vm_pgtable: Replace WRITE_ONCE() with pxd_clear()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/27/26 07:12, Anshuman Khandual wrote:
> Replace WRITE_ONCE() with generic pxd_clear() to clear out the page table
> entries as required. Besides this does not cause any functional change as
> well.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> Applies on mm-unstable and tested only on arm64 platform.
> 
>  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 83cf07269f13..23dc3ee09561 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));
> +	pmd_clear(args->pmdp);
>  	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));
> +	pud_clear(args->pudp);
>  	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);

On some architectures, pmd_clear() does not set the pmd to zero.

See s390x as an example:

	set_pmd(pmdp, __pmd(_SEGMENT_ENTRY_EMPTY));

Which results on 0x20 being set.

Maybe we're lucky and all relevant architectures do not have
CONFIG_HAVE_ARCH_HUGE_VMAP.

Or maybe it just doesn't matter.

We'll find out :)

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David