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

[Anshuman Khandual]

On 26/02/26 6:49 PM, Ryan Roberts wrote:
> On 26/02/2026 12:54, Anshuman Khandual wrote:
>>
>>
>> On 26/02/26 6:07 PM, Usama Arif wrote:
>>> On Tue, 24 Feb 2026 10:41:47 +0530 Anshuman Khandual <anshuman.khandual at 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 at arm.com>
>>>> Cc: Will Deacon <will at kernel.org>
>>>> Cc: Ryan Roberts <ryan.roberts at arm.com>
>>>> Cc: Mark Rutland <mark.rutland at arm.com>
>>>> Cc: linux-arm-kernel at lists.infradead.org
>>>> Cc: linux-kernel at vger.kernel.org
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual at 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.
> 
> I don't think there is any need for WRITE_ONCE() in this file? (and I feel like
> this has come up before in some other context). Could you just change it to a
> plain C assignment? Or even better, pmd_clear()?

Agreed and just sent a patch in this regard.

https://lore.kernel.org/all/20260227061204.2215395-1-anshuman.khandual@arm.com/

> 
>>
>>>
>>>>  	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
>>>>
>>>>
>>>
>>
> 
>