Re: [RFC V1 10/16] arm64/mm: Route all pgtable writes via ptdesc_set()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 26/02/26 6:07 PM, Usama Arif wrote:
> On Tue, 24 Feb 2026 10:41:47 +0530 Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>> Currently ptdesc_set() is defined as WRITE_ONCE() but this will change for
>> D128 pgtable builds, for which WRITE_ONCE() is not sufficient for single
>> copy atomicity.
>>
>> In future this infrastructure can be used for D128 to maintain single copy
>> atomicity semantics with inline asm blocks.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>>  arch/arm64/include/asm/pgtable.h | 11 ++++++-----
>>  arch/arm64/mm/mmu.c              |  4 ++--
>>  mm/debug_vm_pgtable.c            |  4 ++--
>>  3 files changed, 10 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 804ef49aea88..42124d2f323d 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -85,6 +85,7 @@ static inline void arch_leave_lazy_mmu_mode(void)
>>  }
>>  
>>  #define ptdesc_get(x)		READ_ONCE(x)
>> +#define ptdesc_set(x, val)	WRITE_ONCE(x, val)
>>  
>>  #define pmdp_get pmdp_get
>>  static inline pmd_t pmdp_get(pmd_t *pmdp)
>> @@ -389,7 +390,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
>>  
>>  static inline void __set_pte_nosync(pte_t *ptep, pte_t pte)
>>  {
>> -	WRITE_ONCE(*ptep, pte);
>> +	ptdesc_set(*ptep, pte);
>>  }
>>  
>>  static inline void __set_pte_complete(pte_t pte)
>> @@ -856,7 +857,7 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
>>  	}
>>  #endif /* __PAGETABLE_PMD_FOLDED */
>>  
>> -	WRITE_ONCE(*pmdp, pmd);
>> +	ptdesc_set(*pmdp, pmd);
>>  
>>  	if (pmd_valid(pmd))
>>  		queue_pte_barriers();
>> @@ -917,7 +918,7 @@ static inline void set_pud(pud_t *pudp, pud_t pud)
>>  		return;
>>  	}
>>  
>> -	WRITE_ONCE(*pudp, pud);
>> +	ptdesc_set(*pudp, pud);
>>  
>>  	if (pud_valid(pud))
>>  		queue_pte_barriers();
>> @@ -999,7 +1000,7 @@ static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
>>  		return;
>>  	}
>>  
>> -	WRITE_ONCE(*p4dp, p4d);
>> +	ptdesc_set(*p4dp, p4d);
>>  	queue_pte_barriers();
>>  }
>>  
>> @@ -1120,7 +1121,7 @@ static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
>>  		return;
>>  	}
>>  
>> -	WRITE_ONCE(*pgdp, pgd);
>> +	ptdesc_set(*pgdp, pgd);
>>  	queue_pte_barriers();
>>  }
>>  
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index bcf32d1a92de..ffd307c546f5 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -83,7 +83,7 @@ void noinstr set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>>  	 * writable in the kernel mapping.
>>  	 */
>>  	if (rodata_is_rw) {
>> -		WRITE_ONCE(*pgdp, pgd);
>> +		ptdesc_set(*pgdp, pgd);
>>  		dsb(ishst);
>>  		isb();
>>  		return;
>> @@ -91,7 +91,7 @@ void noinstr set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>>  
>>  	spin_lock(&swapper_pgdir_lock);
>>  	fixmap_pgdp = pgd_set_fixmap(__pa_symbol(pgdp));
>> -	WRITE_ONCE(*fixmap_pgdp, pgd);
>> +	ptdesc_set(*fixmap_pgdp, pgd);
>>  	/*
>>  	 * We need dsb(ishst) here to ensure the page-table-walker sees
>>  	 * our new entry before set_p?d() returns. The fixmap's
>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>> index 83cf07269f13..faf6a19a89a1 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));
>> +	ptdesc_set(*args->pmdp, __pmd(0));
> 
> The ptdesc_set() and ptdesc_get() macros are defined in
> arch/arm64/include/asm/pgtable.h and are arm64-specific. This change is
> in mm/debug_vm_pgtable.c which is generic code compiled for all
> architectures. Other architectures do not define ptdesc_set(), so this
> will would cause a build failure on other architectures..

Agreed. Probably will add a local fallback for ptdesc_set() which
defaults to WRITE_ONCE() for platforms not overriding. OR does it
require making ptdesc_get/set() much more generic construct ?

Probably missed this problem while building on some other platforms
as DEBUG_VM_PGTABLE might not have been enabled in their defconfig.

> 
>>  	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));
>> +	ptdesc_set(*args->pudp, __pud(0));
>>  	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);
>> -- 
>> 2.43.0
>>
>>
>