Re: [PATCH V2 1/2] arm64/mm: Move __check_safe_pte_update()

On Wed, Sep 09, 2026 at 05:12:24PM +0200, David Hildenbrand (Arm) wrote:

On 9/9/26 06:00, Anshuman Khandual wrote:

quoted

The page table entry print helpers and related macros which are defined in
<linux/pgtable.h> will not be accessible in platform <asm/pgtable.h> which
is basically caused by cycling dependency.

Move __check_safe_pte_update() inside arch/arm64/mm/mmu.c as a preparation
for subsequent usage of the afore mentioned generic MM helpers. While here
drop IS_ENABLED(CONFIG_DEBUG_VM), although wrap __check_safe_pte_update()
inside #ifdef CONFIG_DEBUG_VM that preserves the current code optimization
which is achieved via the static inline functions.

This does not cause any functional change.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <redacted>
---
Changes in V2:

- Moved __check_safe_pte_update() inside #ifdef CONFIG_DEBUG_VM
- Added empty stub for __check_safe_pte_update() when !CONFIG_DEBUG_VM
- Dropped IS_ENABLED(CONFIG_DEBUG_VM) from __check_safe_pte_update()

 arch/arm64/include/asm/pgtable.h | 49 ++++----------------------------
 arch/arm64/mm/mmu.c              | 44 ++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 44 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index e89ec5f4787b..281a512f1fc6 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -387,52 +387,13 @@ static inline pte_t __ptep_get(pte_t *ptep)
 extern void __sync_icache_dcache(pte_t pteval);
 bool pgattr_change_is_safe(pteval_t old, pteval_t new);
 
-/*
- * PTE bits configuration in the presence of hardware Dirty Bit Management
- * (PTE_WRITE == PTE_DBM):
- *
- * Dirty  Writable | PTE_RDONLY  PTE_WRITE  PTE_DIRTY (sw)
- *   0      0      |   1           0          0
- *   0      1      |   1           1          0
- *   1      0      |   1           0          1
- *   1      1      |   0           1          x
- *
- * When hardware DBM is not present, the software PTE_DIRTY bit is updated via
- * the page fault mechanism. Checking the dirty status of a pte becomes:
- *
- *   PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
- */
-
-static inline void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep,
-					   pte_t pte)
+#ifdef CONFIG_DEBUG_VM
+void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep, pte_t pte);
+#else
+static inline void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep, pte_t pte)
 {
-	pte_t old_pte;
-
-	if (!IS_ENABLED(CONFIG_DEBUG_VM))
-		return;
-
-	old_pte = __ptep_get(ptep);
-
-	if (!pte_valid(old_pte) || !pte_valid(pte))
-		return;
-	if (mm != current->active_mm && atomic_read(&mm->mm_users) <= 1)
-		return;
-
-	/*
-	 * Check for potential race with hardware updates of the pte
-	 * (__ptep_set_access_flags safely changes valid ptes without going
-	 * through an invalid entry).
-	 */
-	VM_WARN_ONCE(!pte_young(pte),
-		     "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
-		     __func__, pte_val(old_pte), pte_val(pte));
-	VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
-		     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
-		     __func__, pte_val(old_pte), pte_val(pte));
-	VM_WARN_ONCE(!pgattr_change_is_safe(pte_val(old_pte), pte_val(pte)),
-		     "%s: unsafe attribute change: 0x%016llx -> 0x%016llx",
-		     __func__, pte_val(old_pte), pte_val(pte));
 }
+#endif

[...]

quoted

+#ifdef CONFIG_DEBUG_VM
+void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep, pte_t pte)
+{
+	pte_t old_pte;
+
+	old_pte = __ptep_get(ptep);
+
+	if (!pte_valid(old_pte) || !pte_valid(pte))
+		return;
+	if (mm != current->active_mm && atomic_read(&mm->mm_users) <= 1)
+		return;
+
+	/*
+	 * Check for potential race with hardware updates of the pte
+	 * (__ptep_set_access_flags safely changes valid ptes without going
+	 * through an invalid entry).
+	 */
+	VM_WARN_ONCE(!pte_young(pte),
+		     "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
+		     __func__, pte_val(old_pte), pte_val(pte));
+	VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
+		     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
+		     __func__, pte_val(old_pte), pte_val(pte));
+	VM_WARN_ONCE(!pgattr_change_is_safe(pte_val(old_pte), pte_val(pte)),
+		     "%s: unsafe attribute change: 0x%016llx -> 0x%016llx",
+		     __func__, pte_val(old_pte), pte_val(pte));
+}
+#endif

#endif /* CONFIG_DEBUG_VM */

Might be helpful here.

Consider the same in the case above

Sure will update the same both in here and also for the declrations
in the header file as well.

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

Thanks for the review.

-- 
Cheers,

David