On 26/02/2026 12:54, Anshuman Khandual wrote:

On 26/02/26 6:07 PM, Usama Arif wrote:

quoted

On Tue, 24 Feb 2026 10:41:47 +0530 Anshuman Khandual [off-list ref] wrote:

quoted

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 <redacted>
---
 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()?

quoted

quoted

 	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