[PATCH v2 0/6] mm: add basic PTE const type-safety

[PATCH v2 0/6] mm: add basic PTE const type-safety

[Pedro Falcato]

Since forever, MM code has thrown pte_t * around with no concern for const
safety, or typesafety of any kind. This is confusing. Attempt to address it
by:
1) Making sure pte_get*() helpers can cope with const pte_t * arguments
2) Constifying the pte_offset_map_ro_nolock() return type, which by definition
already pledges that users will not write to it.

These two simple steps were already able to uncover code smell from
khugepaged + do_swap_page().

Separate steps could include introducing pte_offset_map_ro_lock() for more
widespread usage of this.

Benefits of this include less confusion and better type-safety. It could also
futurely aid in efforts such as [0] which may want semantic annotation of these
accesses.

Based on mm-unstable and compile-tested on a handful of architectures.

No functional changes intended.

[CC list editorially trimmed for brevity reasons; apologies if you're not on it]

Link: https://lore.kernel.org/linux-mm/20260526-kpkeys-v8-0-eaaacdacc67c@arm.com/#t [0]
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-mm@kvack.org
Cc: linux-fsdevel@vger.kernel.org

v2:
 - Small fixups on the arm64 side
 - Re-order patches in a way such that bisection is preserved
 - Pick up Helge's patch dropping parisc ptep_get()
 - Constify s390's ptep_get() as well

Helge Deller (1):
  parisc: Drop own implementations for ptep_get() and
    ptep_test_and_clear_young()

Pedro Falcato (5):
  mm/arm64: constify pte_get*() and contpte get logic
  mm/powerpc/8xx: constify ptep_get() argument
  mm/s390: constify ptep_get() argument
  mm: constify generic pte_get*()
  mm: constify the pte_offset_map_ro_nolock() return value

 arch/arm64/include/asm/pgtable.h             | 10 +++++-----
 arch/arm64/mm/contpte.c                      | 11 ++++++++---
 arch/parisc/include/asm/pgtable.h            | 20 --------------------
 arch/powerpc/include/asm/nohash/32/pte-8xx.h |  2 +-
 arch/powerpc/mm/pgtable.c                    |  2 +-
 arch/s390/include/asm/pgtable.h              |  2 +-
 include/linux/mm.h                           |  4 ++--
 include/linux/pgtable.h                      |  8 ++++----
 mm/filemap.c                                 |  2 +-
 mm/khugepaged.c                              |  2 +-
 mm/pgtable-generic.c                         |  4 ++--
 11 files changed, 26 insertions(+), 41 deletions(-)

-- 
2.55.0

[PATCH v2 1/6] mm/arm64: constify pte_get*() and contpte get logic

[Pedro Falcato]

None of the contpte code needs write access to the PTEs.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 arch/arm64/include/asm/pgtable.h | 10 +++++-----
 arch/arm64/mm/contpte.c          | 11 ++++++++---
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index a2681d755358..043bc0649cee 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -378,7 +378,7 @@ static inline void __set_pte(pte_t *ptep, pte_t pte)
 	__set_pte_complete(pte);
 }
 
-static inline pte_t __ptep_get(pte_t *ptep)
+static inline pte_t __ptep_get(const pte_t *ptep)
 {
 	return READ_ONCE(*ptep);
 }
@@ -1652,8 +1652,8 @@ extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
 				pte_t *ptep, pte_t pte);
 extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
 				pte_t *ptep, pte_t pte);
-extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
-extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
+extern pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte);
+extern pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep);
 extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
 				pte_t *ptep, pte_t pte, unsigned int nr);
 extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
@@ -1732,7 +1732,7 @@ static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
  */
 
 #define ptep_get ptep_get
-static inline pte_t ptep_get(pte_t *ptep)
+static inline pte_t ptep_get(const pte_t *ptep)
 {
 	pte_t pte = __ptep_get(ptep);
 
@@ -1743,7 +1743,7 @@ static inline pte_t ptep_get(pte_t *ptep)
 }
 
 #define ptep_get_lockless ptep_get_lockless
-static inline pte_t ptep_get_lockless(pte_t *ptep)
+static inline pte_t ptep_get_lockless(const pte_t *ptep)
 {
 	pte_t pte = __ptep_get(ptep);
 
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index 2de12656b4d8..3a5d6937fb51 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -26,6 +26,11 @@ static inline pte_t *contpte_align_down(pte_t *ptep)
 	return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
 }
 
+#define contpte_align_down(ptep)                                                       \
+	_Generic((ptep),                                                               \
+		 const pte_t *: (const pte_t *) contpte_align_down((pte_t *) (ptep)),  \
+		 pte_t *: contpte_align_down((pte_t *) ptep))
+
 static inline pte_t *contpte_align_addr_ptep(unsigned long *start,
 					     unsigned long *end, pte_t *ptep,
 					     unsigned int nr)
@@ -310,7 +315,7 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(__contpte_try_unfold);
 
-pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
+pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte)
 {
 	/*
 	 * Gather access/dirty bits, which may be populated in any of the ptes
@@ -367,7 +372,7 @@ static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn,
 			pgprot_val(prot) == pgprot_val(orig_prot);
 }
 
-pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
+pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep)
 {
 	/*
 	 * The ptep_get_lockless() API requires us to read and return *orig_ptep
@@ -386,10 +391,10 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
 	 * because it is not part of a contpte range.
 	 */
 
+	const pte_t *ptep;
 	pgprot_t orig_prot;
 	unsigned long pfn;
 	pte_t orig_pte;
-	pte_t *ptep;
 	pte_t pte;
 	int i;
 
-- 
2.55.0

[PATCH v2 2/6] parisc: Drop own implementations for ptep_get() and ptep_test_and_clear_young()

[Pedro Falcato]

From: Helge Deller <deller@gmx.de>

Switch to the generic implementations, which are identical.

Suggested-by: Usama Arif <usama.arif@linux.dev>
Suggested-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 arch/parisc/include/asm/pgtable.h | 20 --------------------
 1 file changed, 20 deletions(-)

diff --git a/arch/parisc/include/asm/pgtable.h b/arch/parisc/include/asm/pgtable.h
index 467b8547ac8b..d92dee9fef3f 100644
--- a/arch/parisc/include/asm/pgtable.h
+++ b/arch/parisc/include/asm/pgtable.h
@@ -432,25 +432,6 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
 	return pte;
 }
 
-static inline pte_t ptep_get(pte_t *ptep)
-{
-	return READ_ONCE(*ptep);
-}
-#define ptep_get ptep_get
-
-static inline bool ptep_test_and_clear_young(struct vm_area_struct *vma,
-		unsigned long addr, pte_t *ptep)
-{
-	pte_t pte;
-
-	pte = ptep_get(ptep);
-	if (!pte_young(pte)) {
-		return false;
-	}
-	set_pte_at(vma->vm_mm, addr, ptep, pte_mkold(pte));
-	return true;
-}
-
 bool ptep_clear_flush_young(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep);
 pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep);
 
@@ -491,7 +472,6 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr,
 #define HAVE_ARCH_UNMAPPED_AREA
 #define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
 
-#define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
 #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
 #define __HAVE_ARCH_PTEP_CLEAR_FLUSH
 #define __HAVE_ARCH_PTEP_SET_WRPROTECT
-- 
2.55.0

[PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Pedro Falcato]

There is no need for write access to the PTE.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
index e2ea8ba9f8ca..f2bab31040c7 100644
--- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
+++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
@@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
 
 #ifdef CONFIG_PPC_16K_PAGES
 #define ptep_get ptep_get
-static inline pte_t ptep_get(pte_t *ptep)
+static inline pte_t ptep_get(const pte_t *ptep)
 {
 	pte_basic_t val = READ_ONCE(ptep->pte);
 	pte_t pte = {val, val, val, val};
-- 
2.55.0

[PATCH v2 4/6] mm/s390: constify ptep_get() argument

[Pedro Falcato]

It's a simple READ_ONCE() dereference, therefore it can be const.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 arch/s390/include/asm/pgtable.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
index e882663a58e7..ec331ec92943 100644
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -986,7 +986,7 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
 }
 
 #define ptep_get ptep_get
-static inline pte_t ptep_get(pte_t *ptep)
+static inline pte_t ptep_get(const pte_t *ptep)
 {
 	return READ_ONCE(*ptep);
 }
-- 
2.55.0

[PATCH v2 5/6] mm: constify generic pte_get*()

[Pedro Falcato]

None of the helpers need write access to the PTE. Constifying the param
allows for const typesafety.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 include/linux/pgtable.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 8c093c119e5a..dc418553e57a 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -490,7 +490,7 @@ static inline int pudp_set_access_flags(struct vm_area_struct *vma,
 #endif
 
 #ifndef ptep_get
-static inline pte_t ptep_get(pte_t *ptep)
+static inline pte_t ptep_get(const pte_t *ptep)
 {
 	return READ_ONCE(*ptep);
 }
@@ -739,7 +739,7 @@ static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
  * present bit set *unless* it is 'l'. Because get_user_pages_fast() only
  * operates on present ptes we're safe.
  */
-static inline pte_t ptep_get_lockless(pte_t *ptep)
+static inline pte_t ptep_get_lockless(const pte_t *ptep)
 {
 	pte_t pte;
 
@@ -777,7 +777,7 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp)
  * We require that the PTE can be read atomically.
  */
 #ifndef ptep_get_lockless
-static inline pte_t ptep_get_lockless(pte_t *ptep)
+static inline pte_t ptep_get_lockless(const pte_t *ptep)
 {
 	return ptep_get(ptep);
 }
-- 
2.55.0

[PATCH v2 6/6] mm: constify the pte_offset_map_ro_nolock() return value

[Pedro Falcato]

Constify the pte_t * retval from pte_offset_map_ro_nolock(), for which it is
already pledged that accesses must be read-only. With it, convert the three
treewide users to use const pte_t *.

khugepaged passes the result right down to fault code (do_swap_page()). This
leads to a complicated set of conditions that, in order to be correct, must
not install anything into *vmf->pte. This is not trivial to work around in
fault code, and as such just trivially cast to non-const pte_t* in the
meantime.

The other users are far more trivial and the conversion is equally
trivially simple.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 arch/powerpc/mm/pgtable.c | 2 +-
 include/linux/mm.h        | 4 ++--
 include/linux/pgtable.h   | 2 +-
 mm/filemap.c              | 2 +-
 mm/khugepaged.c           | 2 +-
 mm/pgtable-generic.c      | 4 ++--
 6 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index a9be337be3e4..e29db41b6043 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -390,7 +390,7 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
 	p4d_t *p4d;
 	pud_t *pud;
 	pmd_t *pmd;
-	pte_t *pte;
+	const pte_t *pte;
 	spinlock_t *ptl;
 
 	if (mm == &init_mm)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7fabe6c66b4b..acf5a5e31d34 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3885,8 +3885,8 @@ static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr)
 pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
 			   unsigned long addr, spinlock_t **ptlp);
 
-pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
-				unsigned long addr, spinlock_t **ptlp);
+const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
+				      unsigned long addr, spinlock_t **ptlp);
 pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
 				unsigned long addr, pmd_t *pmdvalp,
 				spinlock_t **ptlp);
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index dc418553e57a..dd51e722c535 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -112,7 +112,7 @@ static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address)
 {
 	return pte_offset_kernel(pmd, address);
 }
-static inline void pte_unmap(pte_t *pte)
+static inline void pte_unmap(const pte_t *pte)
 {
 	rcu_read_unlock();
 }
diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec636881f..af5d3fcd1b05 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3490,7 +3490,7 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	vm_fault_t ret = 0;
-	pte_t *ptep;
+	const pte_t *ptep;
 
 	/*
 	 * We might have COW'ed a pagecache folio and might now have an mlocked
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b237f6e7662a..09efac93a8c6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1170,7 +1170,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 			 * Here the ptl is only used to check pte_same() in
 			 * do_swap_page(), so readonly version is enough.
 			 */
-			pte = pte_offset_map_ro_nolock(mm, pmd, addr, &ptl);
+			pte = (pte_t *) pte_offset_map_ro_nolock(mm, pmd, addr, &ptl);
 			if (!pte) {
 				mmap_read_unlock(mm);
 				result = SCAN_NO_PTE_TABLE;
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index b91b1a98029c..2cfc6e608ef4 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -308,8 +308,8 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
 	return NULL;
 }
 
-pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
-				unsigned long addr, spinlock_t **ptlp)
+const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
+				      unsigned long addr, spinlock_t **ptlp)
 {
 	pmd_t pmdval;
 	pte_t *pte;
-- 
2.55.0

Re: [PATCH v2 0/6] mm: add basic PTE const type-safety

[Muhammad Usama Anjum]

On 03/08/2026 5:43 pm, Pedro Falcato wrote:
> Since forever, MM code has thrown pte_t * around with no concern for const
> safety, or typesafety of any kind. This is confusing. Attempt to address it
> by:
> 1) Making sure pte_get*() helpers can cope with const pte_t * arguments
> 2) Constifying the pte_offset_map_ro_nolock() return type, which by definition
> already pledges that users will not write to it.
> 
> These two simple steps were already able to uncover code smell from
> khugepaged + do_swap_page().
> 
> Separate steps could include introducing pte_offset_map_ro_lock() for more
> widespread usage of this.
> 
> Benefits of this include less confusion and better type-safety. It could also
> futurely aid in efforts such as [0] which may want semantic annotation of these
> accesses.
> 
> Based on mm-unstable and compile-tested on a handful of architectures.
> 
> No functional changes intended.
I've reviewed the entire series. Hence:

Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>

> 
> [CC list editorially trimmed for brevity reasons; apologies if you're not on it]
> 
> Link: https://lore.kernel.org/linux-mm/20260526-kpkeys-v8-0-eaaacdacc67c@arm.com/#t [0]
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
> Cc: Helge Deller <deller@gmx.de>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: "Liam R. Howlett" <liam@infradead.org>
> Cc: Vlastimil Babka <vbabka@kernel.org>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Usama Arif <usama.arif@linux.dev>
> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
> Cc: Muhammad Usama Anjum <usama.anjum@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-parisc@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-mm@kvack.org
> Cc: linux-fsdevel@vger.kernel.org
> 
> v2:
>  - Small fixups on the arm64 side
>  - Re-order patches in a way such that bisection is preserved
>  - Pick up Helge's patch dropping parisc ptep_get()
>  - Constify s390's ptep_get() as well
> 
> Helge Deller (1):
>   parisc: Drop own implementations for ptep_get() and
>     ptep_test_and_clear_young()
> 
> Pedro Falcato (5):
>   mm/arm64: constify pte_get*() and contpte get logic
>   mm/powerpc/8xx: constify ptep_get() argument
>   mm/s390: constify ptep_get() argument
>   mm: constify generic pte_get*()
>   mm: constify the pte_offset_map_ro_nolock() return value
> 
>  arch/arm64/include/asm/pgtable.h             | 10 +++++-----
>  arch/arm64/mm/contpte.c                      | 11 ++++++++---
>  arch/parisc/include/asm/pgtable.h            | 20 --------------------
>  arch/powerpc/include/asm/nohash/32/pte-8xx.h |  2 +-
>  arch/powerpc/mm/pgtable.c                    |  2 +-
>  arch/s390/include/asm/pgtable.h              |  2 +-
>  include/linux/mm.h                           |  4 ++--
>  include/linux/pgtable.h                      |  8 ++++----
>  mm/filemap.c                                 |  2 +-
>  mm/khugepaged.c                              |  2 +-
>  mm/pgtable-generic.c                         |  4 ++--
>  11 files changed, 26 insertions(+), 41 deletions(-)
> 

-- 
Thanks,
Usama

Re: [PATCH v2 0/6] mm: add basic PTE const type-safety

[Christophe Leroy (CS GROUP)]

Le 03/08/2026 à 18:43, Pedro Falcato a écrit :
> Since forever, MM code has thrown pte_t * around with no concern for const
> safety, or typesafety of any kind. This is confusing. Attempt to address it
> by:

What do you mean by "typesafety of any kind" ?

On powerpc64, pte_t is a struct so you can't play-up too much with it.

On powerpc32, pte_t is a long int because having it as a struct is 
counter-performant, but we have it as a struct when __CHECKER__ is 
defined, ie when doing a sparse check with 'make C=2'.


> 1) Making sure pte_get*() helpers can cope with const pte_t * arguments
> 2) Constifying the pte_offset_map_ro_nolock() return type, which by definition
> already pledges that users will not write to it.
> 
> These two simple steps were already able to uncover code smell from
> khugepaged + do_swap_page().
> 
> Separate steps could include introducing pte_offset_map_ro_lock() for more
> widespread usage of this.
> 
> Benefits of this include less confusion and better type-safety. It could also
> futurely aid in efforts such as [0] which may want semantic annotation of these
> accesses.
> 
> Based on mm-unstable and compile-tested on a handful of architectures.
> 
> No functional changes intended.
> 

For the series,

Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>



> [CC list editorially trimmed for brevity reasons; apologies if you're not on it]
> 
> Link: https://lore.kernel.org/linux-mm/20260526-kpkeys-v8-0-eaaacdacc67c@arm.com/#t [0]
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
> Cc: Helge Deller <deller@gmx.de>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: "Liam R. Howlett" <liam@infradead.org>
> Cc: Vlastimil Babka <vbabka@kernel.org>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Usama Arif <usama.arif@linux.dev>
> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
> Cc: Muhammad Usama Anjum <usama.anjum@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-parisc@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-mm@kvack.org
> Cc: linux-fsdevel@vger.kernel.org
> 
> v2:
>   - Small fixups on the arm64 side
>   - Re-order patches in a way such that bisection is preserved
>   - Pick up Helge's patch dropping parisc ptep_get()
>   - Constify s390's ptep_get() as well
> 
> Helge Deller (1):
>    parisc: Drop own implementations for ptep_get() and
>      ptep_test_and_clear_young()
> 
> Pedro Falcato (5):
>    mm/arm64: constify pte_get*() and contpte get logic
>    mm/powerpc/8xx: constify ptep_get() argument
>    mm/s390: constify ptep_get() argument
>    mm: constify generic pte_get*()
>    mm: constify the pte_offset_map_ro_nolock() return value
> 
>   arch/arm64/include/asm/pgtable.h             | 10 +++++-----
>   arch/arm64/mm/contpte.c                      | 11 ++++++++---
>   arch/parisc/include/asm/pgtable.h            | 20 --------------------
>   arch/powerpc/include/asm/nohash/32/pte-8xx.h |  2 +-
>   arch/powerpc/mm/pgtable.c                    |  2 +-
>   arch/s390/include/asm/pgtable.h              |  2 +-
>   include/linux/mm.h                           |  4 ++--
>   include/linux/pgtable.h                      |  8 ++++----
>   mm/filemap.c                                 |  2 +-
>   mm/khugepaged.c                              |  2 +-
>   mm/pgtable-generic.c                         |  4 ++--
>   11 files changed, 26 insertions(+), 41 deletions(-)
> 

Re: [PATCH v2 1/6] mm/arm64: constify pte_get*() and contpte get logic

[Lorenzo Stoakes (ARM)]

On Mon, Aug 03, 2026 at 05:43:55PM +0100, Pedro Falcato wrote:
> None of the contpte code needs write access to the PTEs.

This seems like a very broad statement? Is that actually true? I see a bunch of
ptep's that aren't const-ified, so you should explain why those couldn't be
converted.

Also you add a new contpte_align_down() macro, you should mention that it the
commit message, explain why it was needed.

In general more needed here :) it'd be ok if it was a truly trivial change that
was all obvious but you're changing some pte_t *'s and not others so it's
clearly not.

>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> ---
>  arch/arm64/include/asm/pgtable.h | 10 +++++-----
>  arch/arm64/mm/contpte.c          | 11 ++++++++---
>  2 files changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index a2681d755358..043bc0649cee 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -378,7 +378,7 @@ static inline void __set_pte(pte_t *ptep, pte_t pte)
>  	__set_pte_complete(pte);
>  }
>
> -static inline pte_t __ptep_get(pte_t *ptep)
> +static inline pte_t __ptep_get(const pte_t *ptep)
>  {
>  	return READ_ONCE(*ptep);
>  }
> @@ -1652,8 +1652,8 @@ extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
>  				pte_t *ptep, pte_t pte);
>  extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>  				pte_t *ptep, pte_t pte);
> -extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
> -extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
> +extern pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte);
> +extern pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep);

This is (very) nitty but - not sure on the policy on extern's (Will/Catalin?) -
but in mm we drop them when we touch the code since you don't need them these
days :)

>  extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
>  				pte_t *ptep, pte_t pte, unsigned int nr);
>  extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> @@ -1732,7 +1732,7 @@ static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
>   */
>
>  #define ptep_get ptep_get
> -static inline pte_t ptep_get(pte_t *ptep)
> +static inline pte_t ptep_get(const pte_t *ptep)
>  {
>  	pte_t pte = __ptep_get(ptep);
>
> @@ -1743,7 +1743,7 @@ static inline pte_t ptep_get(pte_t *ptep)
>  }
>
>  #define ptep_get_lockless ptep_get_lockless
> -static inline pte_t ptep_get_lockless(pte_t *ptep)
> +static inline pte_t ptep_get_lockless(const pte_t *ptep)
>  {
>  	pte_t pte = __ptep_get(ptep);
>
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> index 2de12656b4d8..3a5d6937fb51 100644
> --- a/arch/arm64/mm/contpte.c
> +++ b/arch/arm64/mm/contpte.c
> @@ -26,6 +26,11 @@ static inline pte_t *contpte_align_down(pte_t *ptep)
>  	return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
>  }
>
> +#define contpte_align_down(ptep)                                                       \
> +	_Generic((ptep),                                                               \
> +		 const pte_t *: (const pte_t *) contpte_align_down((pte_t *) (ptep)),  \
> +		 pte_t *: contpte_align_down((pte_t *) ptep))

I really hate these _Generic() helper things. So ugly. And it's a pretty horrid
cast now :(

Was it not possible to const-ify further to just be able to constify
contpte_align_down itself?

It also seems to contradict the claim that contpte doesn't need write-access to
pte's since you're going to lengths to allow non-const pte_t * here.

You should cover off why this was necessary in the commit message as above.

Anyway PTR_ALIGN_DOWN() is already const-safe so couldn't you anyway just
collapse this to:

#define contpte_align_down(ptep) \
	PTR_ALIGN_DOWN(ptep, sizeof(*(ptep)) * CONT_PTES)

Then describe in the commit message why you need to handle both cases?

> +
>  static inline pte_t *contpte_align_addr_ptep(unsigned long *start,
>  					     unsigned long *end, pte_t *ptep,
>  					     unsigned int nr)
> @@ -310,7 +315,7 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>  }
>  EXPORT_SYMBOL_GPL(__contpte_try_unfold);
>
> -pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> +pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte)
>  {
>  	/*
>  	 * Gather access/dirty bits, which may be populated in any of the ptes
> @@ -367,7 +372,7 @@ static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn,
>  			pgprot_val(prot) == pgprot_val(orig_prot);
>  }
>
> -pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> +pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep)
>  {
>  	/*
>  	 * The ptep_get_lockless() API requires us to read and return *orig_ptep
> @@ -386,10 +391,10 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
>  	 * because it is not part of a contpte range.
>  	 */
>
> +	const pte_t *ptep;

Nit but this is breaking the reverse xmas tree isn't it?

>  	pgprot_t orig_prot;
>  	unsigned long pfn;
>  	pte_t orig_pte;
> -	pte_t *ptep;
>  	pte_t pte;
>  	int i;
>
> --
> 2.55.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 2/6] parisc: Drop own implementations for ptep_get() and ptep_test_and_clear_young()

[Lorenzo Stoakes (ARM)]

On Mon, Aug 03, 2026 at 05:43:56PM +0100, Pedro Falcato wrote:
> From: Helge Deller <deller@gmx.de>
>
> Switch to the generic implementations, which are identical.

You sure do like succinct commit messages :)

Maybe worth saying by dropping the __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG and
ptep_get defines you get the generic versions from include/pgtable.h which are
functionally identical.

(Being pedantic, they're not quite strictly identical as the
ptep_test_and_clear_young() generic function does some weird unnecessary
indirection with a local variable and the single {} is dropped etc.)

>
> Suggested-by: Usama Arif <usama.arif@linux.dev>
> Suggested-by: John David Anglin <dave.anglin@bell.net>
> Signed-off-by: Helge Deller <deller@gmx.de>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>

Nits above notwithstanding, LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  arch/parisc/include/asm/pgtable.h | 20 --------------------
>  1 file changed, 20 deletions(-)
>
> diff --git a/arch/parisc/include/asm/pgtable.h b/arch/parisc/include/asm/pgtable.h
> index 467b8547ac8b..d92dee9fef3f 100644
> --- a/arch/parisc/include/asm/pgtable.h
> +++ b/arch/parisc/include/asm/pgtable.h
> @@ -432,25 +432,6 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
>  	return pte;
>  }
>
> -static inline pte_t ptep_get(pte_t *ptep)
> -{
> -	return READ_ONCE(*ptep);
> -}
> -#define ptep_get ptep_get
> -
> -static inline bool ptep_test_and_clear_young(struct vm_area_struct *vma,
> -		unsigned long addr, pte_t *ptep)
> -{
> -	pte_t pte;
> -
> -	pte = ptep_get(ptep);
> -	if (!pte_young(pte)) {
> -		return false;
> -	}
> -	set_pte_at(vma->vm_mm, addr, ptep, pte_mkold(pte));
> -	return true;
> -}
> -
>  bool ptep_clear_flush_young(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep);
>  pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep);
>
> @@ -491,7 +472,6 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr,
>  #define HAVE_ARCH_UNMAPPED_AREA
>  #define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
>
> -#define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
>  #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
>  #define __HAVE_ARCH_PTEP_CLEAR_FLUSH
>  #define __HAVE_ARCH_PTEP_SET_WRPROTECT
> --
> 2.55.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Lorenzo Stoakes (ARM)]

On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
> There is no need for write access to the PTE.
>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>

With below addressed LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> index e2ea8ba9f8ca..f2bab31040c7 100644
> --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
>
>  #ifdef CONFIG_PPC_16K_PAGES
>  #define ptep_get ptep_get
> -static inline pte_t ptep_get(pte_t *ptep)
> +static inline pte_t ptep_get(const pte_t *ptep)
>  {
>  	pte_basic_t val = READ_ONCE(ptep->pte);
>  	pte_t pte = {val, val, val, val};

Worth making these const too (that {val, val, val, val} horrifies me btw :)?

> --
> 2.55.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 4/6] mm/s390: constify ptep_get() argument

[Lorenzo Stoakes (ARM)]

On Mon, Aug 03, 2026 at 05:43:58PM +0100, Pedro Falcato wrote:
> It's a simple READ_ONCE() dereference, therefore it can be const.
>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>

Even I can't be pedantic about this one... :) so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  arch/s390/include/asm/pgtable.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index e882663a58e7..ec331ec92943 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -986,7 +986,7 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
>  }
>
>  #define ptep_get ptep_get
> -static inline pte_t ptep_get(pte_t *ptep)
> +static inline pte_t ptep_get(const pte_t *ptep)
>  {
>  	return READ_ONCE(*ptep);
>  }
> --
> 2.55.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 5/6] mm: constify generic pte_get*()

[Lorenzo Stoakes (ARM)]

On Mon, Aug 03, 2026 at 05:43:59PM +0100, Pedro Falcato wrote:
> None of the helpers need write access to the PTE. Constifying the param
> allows for const typesafety.

constity?

>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>

LGTM, so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  include/linux/pgtable.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 8c093c119e5a..dc418553e57a 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -490,7 +490,7 @@ static inline int pudp_set_access_flags(struct vm_area_struct *vma,
>  #endif
>
>  #ifndef ptep_get
> -static inline pte_t ptep_get(pte_t *ptep)
> +static inline pte_t ptep_get(const pte_t *ptep)
>  {
>  	return READ_ONCE(*ptep);
>  }
> @@ -739,7 +739,7 @@ static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
>   * present bit set *unless* it is 'l'. Because get_user_pages_fast() only
>   * operates on present ptes we're safe.
>   */
> -static inline pte_t ptep_get_lockless(pte_t *ptep)
> +static inline pte_t ptep_get_lockless(const pte_t *ptep)
>  {
>  	pte_t pte;
>
> @@ -777,7 +777,7 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp)
>   * We require that the PTE can be read atomically.
>   */
>  #ifndef ptep_get_lockless
> -static inline pte_t ptep_get_lockless(pte_t *ptep)
> +static inline pte_t ptep_get_lockless(const pte_t *ptep)
>  {
>  	return ptep_get(ptep);
>  }
> --
> 2.55.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 6/6] mm: constify the pte_offset_map_ro_nolock() return value

[Lorenzo Stoakes (ARM)]

On Mon, Aug 03, 2026 at 05:44:00PM +0100, Pedro Falcato wrote:
> Constify the pte_t * retval from pte_offset_map_ro_nolock(), for which it is
> already pledged that accesses must be read-only. With it, convert the three
> treewide users to use const pte_t *.
>
> khugepaged passes the result right down to fault code (do_swap_page()). This
> leads to a complicated set of conditions that, in order to be correct, must
> not install anything into *vmf->pte. This is not trivial to work around in
> fault code, and as such just trivially cast to non-const pte_t* in the
> meantime.
>
> The other users are far more trivial and the conversion is equally
> trivially simple.

Ah finally more words! :)

>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>

With comment updated as below and nits addressed, LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  arch/powerpc/mm/pgtable.c | 2 +-
>  include/linux/mm.h        | 4 ++--
>  include/linux/pgtable.h   | 2 +-
>  mm/filemap.c              | 2 +-
>  mm/khugepaged.c           | 2 +-
>  mm/pgtable-generic.c      | 4 ++--
>  6 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
> index a9be337be3e4..e29db41b6043 100644
> --- a/arch/powerpc/mm/pgtable.c
> +++ b/arch/powerpc/mm/pgtable.c
> @@ -390,7 +390,7 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
>  	p4d_t *p4d;
>  	pud_t *pud;
>  	pmd_t *pmd;
> -	pte_t *pte;
> +	const pte_t *pte;
>  	spinlock_t *ptl;
>
>  	if (mm == &init_mm)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7fabe6c66b4b..acf5a5e31d34 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3885,8 +3885,8 @@ static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr)
>  pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
>  			   unsigned long addr, spinlock_t **ptlp);
>
> -pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> -				unsigned long addr, spinlock_t **ptlp);
> +const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> +				      unsigned long addr, spinlock_t **ptlp);
>  pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
>  				unsigned long addr, pmd_t *pmdvalp,
>  				spinlock_t **ptlp);
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index dc418553e57a..dd51e722c535 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -112,7 +112,7 @@ static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address)
>  {
>  	return pte_offset_kernel(pmd, address);
>  }
> -static inline void pte_unmap(pte_t *pte)
> +static inline void pte_unmap(const pte_t *pte)

I was going to question this based on whether the contract holds for
CONFIG_HIGHPTE but actually:

#define pte_unmap(pte)	do {	\
	kunmap_local((pte));	\
	rcu_read_unlock();	\
} while (0)

#define kunmap_local(__addr)					\
do {								\
	BUILD_BUG_ON(__same_type((__addr), struct page *));	\
	__kunmap_local(__addr);					\
} while (0)

static inline void __kunmap_local(const void *vaddr) <-- const!
{
	kunmap_local_indexed(vaddr);
}

So nice (CONFIG_HIGHPTE is going to go away at some point though, right? I
hope... :)


>  {
>  	rcu_read_unlock();
>  }
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6afec636881f..af5d3fcd1b05 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3490,7 +3490,7 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	vm_fault_t ret = 0;
> -	pte_t *ptep;
> +	const pte_t *ptep;
>
>  	/*
>  	 * We might have COW'ed a pagecache folio and might now have an mlocked
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b237f6e7662a..09efac93a8c6 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1170,7 +1170,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
>  			 * Here the ptl is only used to check pte_same() in
>  			 * do_swap_page(), so readonly version is enough.
>  			 */
> -			pte = pte_offset_map_ro_nolock(mm, pmd, addr, &ptl);
> +			pte = (pte_t *) pte_offset_map_ro_nolock(mm, pmd, addr, &ptl);

Hmm yeah this is nasty, but you explain why in the commit message. Could you
extend the comment to explain it?

>  			if (!pte) {
>  				mmap_read_unlock(mm);
>  				result = SCAN_NO_PTE_TABLE;
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index b91b1a98029c..2cfc6e608ef4 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -308,8 +308,8 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>  	return NULL;
>  }
>
> -pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> -				unsigned long addr, spinlock_t **ptlp)
> +const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,

Can pmd be const too?

> +				      unsigned long addr, spinlock_t **ptlp)
>  {
>  	pmd_t pmdval;
>  	pte_t *pte;
> --
> 2.55.0
>

--
Cheers, Lorenzo

Re: [PATCH v2 1/6] mm/arm64: constify pte_get*() and contpte get logic

[Pedro Falcato]

On Tue, Aug 04, 2026 at 11:55:09AM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Aug 03, 2026 at 05:43:55PM +0100, Pedro Falcato wrote:
> > None of the contpte code needs write access to the PTEs.
> 
> This seems like a very broad statement? Is that actually true? I see a bunch of
> ptep's that aren't const-ified, so you should explain why those couldn't be
> converted.

ACK. FTR, I think it would've been far clearer with "None of the contpte get
code". There is of course contpte code that needs write access (e.g
contpte_clear_full_ptes).

> 
> Also you add a new contpte_align_down() macro, you should mention that it the
> commit message, explain why it was needed.
> 
> In general more needed here :) it'd be ok if it was a truly trivial change that
> was all obvious but you're changing some pte_t *'s and not others so it's
> clearly not.
> 
> >
> > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 10 +++++-----
> >  arch/arm64/mm/contpte.c          | 11 ++++++++---
> >  2 files changed, 13 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index a2681d755358..043bc0649cee 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -378,7 +378,7 @@ static inline void __set_pte(pte_t *ptep, pte_t pte)
> >  	__set_pte_complete(pte);
> >  }
> >
> > -static inline pte_t __ptep_get(pte_t *ptep)
> > +static inline pte_t __ptep_get(const pte_t *ptep)
> >  {
> >  	return READ_ONCE(*ptep);
> >  }
> > @@ -1652,8 +1652,8 @@ extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
> >  				pte_t *ptep, pte_t pte);
> >  extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> >  				pte_t *ptep, pte_t pte);
> > -extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
> > -extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
> > +extern pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte);
> > +extern pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep);
> 
> This is (very) nitty but - not sure on the policy on extern's (Will/Catalin?) -
> but in mm we drop them when we touch the code since you don't need them these
> days :)

*nods*. For what it's worth, this is new code that never needed extern.

> 
> >  extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> >  				pte_t *ptep, pte_t pte, unsigned int nr);
> >  extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> > @@ -1732,7 +1732,7 @@ static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
> >   */
> >
> >  #define ptep_get ptep_get
> > -static inline pte_t ptep_get(pte_t *ptep)
> > +static inline pte_t ptep_get(const pte_t *ptep)
> >  {
> >  	pte_t pte = __ptep_get(ptep);
> >
> > @@ -1743,7 +1743,7 @@ static inline pte_t ptep_get(pte_t *ptep)
> >  }
> >
> >  #define ptep_get_lockless ptep_get_lockless
> > -static inline pte_t ptep_get_lockless(pte_t *ptep)
> > +static inline pte_t ptep_get_lockless(const pte_t *ptep)
> >  {
> >  	pte_t pte = __ptep_get(ptep);
> >
> > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > index 2de12656b4d8..3a5d6937fb51 100644
> > --- a/arch/arm64/mm/contpte.c
> > +++ b/arch/arm64/mm/contpte.c
> > @@ -26,6 +26,11 @@ static inline pte_t *contpte_align_down(pte_t *ptep)
> >  	return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
> >  }
> >
> > +#define contpte_align_down(ptep)                                                       \
> > +	_Generic((ptep),                                                               \
> > +		 const pte_t *: (const pte_t *) contpte_align_down((pte_t *) (ptep)),  \
> > +		 pte_t *: contpte_align_down((pte_t *) ptep))
> 
> I really hate these _Generic() helper things. So ugly. And it's a pretty horrid
> cast now :(

Me too!

> 
> Was it not possible to const-ify further to just be able to constify
> contpte_align_down itself?
> 

You can't do that because some callers want a pte_t* out of align_down,
others want a const pte_t* out of align_down, depending on the param.

In A More Civilized Language(TM):

template <typename T>
T contpte_align_down(T ptr);

:P

> It also seems to contradict the claim that contpte doesn't need write-access to
> pte's since you're going to lengths to allow non-const pte_t * here.

Yep, I'll admit the commit message is confusing and crap. I'll flesh it out
here.

> 
> You should cover off why this was necessary in the commit message as above.
> 
> Anyway PTR_ALIGN_DOWN() is already const-safe so couldn't you anyway just
> collapse this to:
> 
> #define contpte_align_down(ptep) \
> 	PTR_ALIGN_DOWN(ptep, sizeof(*(ptep)) * CONT_PTES)
> 
> Then describe in the commit message why you need to handle both cases?
> 
> > +
> >  static inline pte_t *contpte_align_addr_ptep(unsigned long *start,
> >  					     unsigned long *end, pte_t *ptep,
> >  					     unsigned int nr)
> > @@ -310,7 +315,7 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> >  }
> >  EXPORT_SYMBOL_GPL(__contpte_try_unfold);
> >
> > -pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> > +pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte)
> >  {
> >  	/*
> >  	 * Gather access/dirty bits, which may be populated in any of the ptes
> > @@ -367,7 +372,7 @@ static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn,
> >  			pgprot_val(prot) == pgprot_val(orig_prot);
> >  }
> >
> > -pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> > +pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep)
> >  {
> >  	/*
> >  	 * The ptep_get_lockless() API requires us to read and return *orig_ptep
> > @@ -386,10 +391,10 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> >  	 * because it is not part of a contpte range.
> >  	 */
> >
> > +	const pte_t *ptep;
> 
> Nit but this is breaking the reverse xmas tree isn't it?

Yep, seems like I mistakenly broke the coding style here and in the
contpte_align_down macro above (the \ is misaligned). I'll fix it up.

(I think Andrew isn't taking more material for next cycle, and while
this should have no functional effect, it is very late and you have
pushback, so probably no rush here...)

-- 
Pedro

Re: [PATCH v2 2/6] parisc: Drop own implementations for ptep_get() and ptep_test_and_clear_young()

[Pedro Falcato]

On Tue, Aug 04, 2026 at 12:11:08PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Aug 03, 2026 at 05:43:56PM +0100, Pedro Falcato wrote:
> > From: Helge Deller <deller@gmx.de>
> >
> > Switch to the generic implementations, which are identical.
> 
> You sure do like succinct commit messages :)

I didn't even write this one! See the From: :))

> 
> Maybe worth saying by dropping the __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG and
> ptep_get defines you get the generic versions from include/pgtable.h which are
> functionally identical.
> 

I can touch it up though, if you insist.

> (Being pedantic, they're not quite strictly identical as the
> ptep_test_and_clear_young() generic function does some weird unnecessary
> indirection with a local variable and the single {} is dropped etc.)
> 
> >
> > Suggested-by: Usama Arif <usama.arif@linux.dev>
> > Suggested-by: John David Anglin <dave.anglin@bell.net>
> > Signed-off-by: Helge Deller <deller@gmx.de>
> > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> 
> Nits above notwithstanding, LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Thanks!

-- 
Pedro

Re: [PATCH v2 1/6] mm/arm64: constify pte_get*() and contpte get logic

[Lorenzo Stoakes (ARM)]

On Tue, Aug 04, 2026 at 01:31:04PM +0100, Pedro Falcato wrote:
> On Tue, Aug 04, 2026 at 11:55:09AM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Aug 03, 2026 at 05:43:55PM +0100, Pedro Falcato wrote:
> > > None of the contpte code needs write access to the PTEs.
> >
> > This seems like a very broad statement? Is that actually true? I see a bunch of
> > ptep's that aren't const-ified, so you should explain why those couldn't be
> > converted.
>
> ACK. FTR, I think it would've been far clearer with "None of the contpte get
> code". There is of course contpte code that needs write access (e.g
> contpte_clear_full_ptes).

yeah that sounds better. Maybe just something like 'specify const for read-only
users of pte *' or similar?

>
> >
> > Also you add a new contpte_align_down() macro, you should mention that it the
> > commit message, explain why it was needed.
> >
> > In general more needed here :) it'd be ok if it was a truly trivial change that
> > was all obvious but you're changing some pte_t *'s and not others so it's
> > clearly not.
> >
> > >
> > > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> > > ---
> > >  arch/arm64/include/asm/pgtable.h | 10 +++++-----
> > >  arch/arm64/mm/contpte.c          | 11 ++++++++---
> > >  2 files changed, 13 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > > index a2681d755358..043bc0649cee 100644
> > > --- a/arch/arm64/include/asm/pgtable.h
> > > +++ b/arch/arm64/include/asm/pgtable.h
> > > @@ -378,7 +378,7 @@ static inline void __set_pte(pte_t *ptep, pte_t pte)
> > >  	__set_pte_complete(pte);
> > >  }
> > >
> > > -static inline pte_t __ptep_get(pte_t *ptep)
> > > +static inline pte_t __ptep_get(const pte_t *ptep)
> > >  {
> > >  	return READ_ONCE(*ptep);
> > >  }
> > > @@ -1652,8 +1652,8 @@ extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
> > >  				pte_t *ptep, pte_t pte);
> > >  extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> > >  				pte_t *ptep, pte_t pte);
> > > -extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
> > > -extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
> > > +extern pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte);
> > > +extern pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep);
> >
> > This is (very) nitty but - not sure on the policy on extern's (Will/Catalin?) -
> > but in mm we drop them when we touch the code since you don't need them these
> > days :)
>
> *nods*. For what it's worth, this is new code that never needed extern.

Yup, this isn't a big deal anyway :)

>
> >
> > >  extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> > >  				pte_t *ptep, pte_t pte, unsigned int nr);
> > >  extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> > > @@ -1732,7 +1732,7 @@ static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
> > >   */
> > >
> > >  #define ptep_get ptep_get
> > > -static inline pte_t ptep_get(pte_t *ptep)
> > > +static inline pte_t ptep_get(const pte_t *ptep)
> > >  {
> > >  	pte_t pte = __ptep_get(ptep);
> > >
> > > @@ -1743,7 +1743,7 @@ static inline pte_t ptep_get(pte_t *ptep)
> > >  }
> > >
> > >  #define ptep_get_lockless ptep_get_lockless
> > > -static inline pte_t ptep_get_lockless(pte_t *ptep)
> > > +static inline pte_t ptep_get_lockless(const pte_t *ptep)
> > >  {
> > >  	pte_t pte = __ptep_get(ptep);
> > >
> > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > > index 2de12656b4d8..3a5d6937fb51 100644
> > > --- a/arch/arm64/mm/contpte.c
> > > +++ b/arch/arm64/mm/contpte.c
> > > @@ -26,6 +26,11 @@ static inline pte_t *contpte_align_down(pte_t *ptep)
> > >  	return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
> > >  }
> > >
> > > +#define contpte_align_down(ptep)                                                       \
> > > +	_Generic((ptep),                                                               \
> > > +		 const pte_t *: (const pte_t *) contpte_align_down((pte_t *) (ptep)),  \
> > > +		 pte_t *: contpte_align_down((pte_t *) ptep))
> >
> > I really hate these _Generic() helper things. So ugly. And it's a pretty horrid
> > cast now :(
>
> Me too!
>
> >
> > Was it not possible to const-ify further to just be able to constify
> > contpte_align_down itself?
> >
>
> You can't do that because some callers want a pte_t* out of align_down,
> others want a const pte_t* out of align_down, depending on the param.
>
> In A More Civilized Language(TM):
>
> template <typename T>
> T contpte_align_down(T ptr);

Now do it in rust ;)

>
> :P

Well as per below you can just avoid the whole issue with:

#define contpte_align_down(ptep) \
 	PTR_ALIGN_DOWN(ptep, sizeof(*(ptep)) * CONT_PTES)

And avoid this mess? You're ultimately calling into a macro anyway.

The generic thing Seems like a lot of hassle for an align-down, and then you're
forcing a macro and nasty casts anyway so I don't really see the downside of
just making it a macro in general?

>
> > It also seems to contradict the claim that contpte doesn't need write-access to
> > pte's since you're going to lengths to allow non-const pte_t * here.
>
> Yep, I'll admit the commit message is confusing and crap. I'll flesh it out
> here.

Thanks!

>
> >
> > You should cover off why this was necessary in the commit message as above.
> >
> > Anyway PTR_ALIGN_DOWN() is already const-safe so couldn't you anyway just
> > collapse this to:
> >
> > #define contpte_align_down(ptep) \
> > 	PTR_ALIGN_DOWN(ptep, sizeof(*(ptep)) * CONT_PTES)
> >
> > Then describe in the commit message why you need to handle both cases?
> >
> > > +
> > >  static inline pte_t *contpte_align_addr_ptep(unsigned long *start,
> > >  					     unsigned long *end, pte_t *ptep,
> > >  					     unsigned int nr)
> > > @@ -310,7 +315,7 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> > >  }
> > >  EXPORT_SYMBOL_GPL(__contpte_try_unfold);
> > >
> > > -pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> > > +pte_t contpte_ptep_get(const pte_t *ptep, pte_t orig_pte)
> > >  {
> > >  	/*
> > >  	 * Gather access/dirty bits, which may be populated in any of the ptes
> > > @@ -367,7 +372,7 @@ static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn,
> > >  			pgprot_val(prot) == pgprot_val(orig_prot);
> > >  }
> > >
> > > -pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> > > +pte_t contpte_ptep_get_lockless(const pte_t *orig_ptep)
> > >  {
> > >  	/*
> > >  	 * The ptep_get_lockless() API requires us to read and return *orig_ptep
> > > @@ -386,10 +391,10 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> > >  	 * because it is not part of a contpte range.
> > >  	 */
> > >
> > > +	const pte_t *ptep;
> >
> > Nit but this is breaking the reverse xmas tree isn't it?
>
> Yep, seems like I mistakenly broke the coding style here and in the
> contpte_align_down macro above (the \ is misaligned). I'll fix it up.
>
> (I think Andrew isn't taking more material for next cycle, and while
> this should have no functional effect, it is very late and you have
> pushback, so probably no rush here...)

Yeah I assumed this was for 7.4 :) I mean the series is fine for 7.3 too AFAIC
(with feedback addressed obviously).

>
> --
> Pedro

--
Cheers, Lorenzo

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Pedro Falcato]

On Tue, Aug 04, 2026 at 12:13:13PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
> > There is no need for write access to the PTE.
> >
> > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> 
> With below addressed LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> 
> > ---
> >  arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > index e2ea8ba9f8ca..f2bab31040c7 100644
> > --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
> >
> >  #ifdef CONFIG_PPC_16K_PAGES
> >  #define ptep_get ptep_get
> > -static inline pte_t ptep_get(pte_t *ptep)
> > +static inline pte_t ptep_get(const pte_t *ptep)
> >  {
> >  	pte_basic_t val = READ_ONCE(ptep->pte);
> >  	pte_t pte = {val, val, val, val};
> 
> Worth making these const too (that {val, val, val, val} horrifies me btw :)?

This is PPC code, so I don't know if they have any particular opinion here,
but I could definitely do this.

(FWIW, it seems we're more aggressive in MM in doing this than other places
in the kernel?)

-- 
Pedro

Re: [PATCH v2 2/6] parisc: Drop own implementations for ptep_get() and ptep_test_and_clear_young()

[Lorenzo Stoakes (ARM)]

On Tue, Aug 04, 2026 at 01:34:41PM +0100, Pedro Falcato wrote:
> On Tue, Aug 04, 2026 at 12:11:08PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Aug 03, 2026 at 05:43:56PM +0100, Pedro Falcato wrote:
> > > From: Helge Deller <deller@gmx.de>
> > >
> > > Switch to the generic implementations, which are identical.
> >
> > You sure do like succinct commit messages :)
>
> I didn't even write this one! See the From: :))

Lol well then feedback addressed to Helge ;)

>
> >
> > Maybe worth saying by dropping the __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG and
> > ptep_get defines you get the generic versions from include/pgtable.h which are
> > functionally identical.
> >
>
> I can touch it up though, if you insist.

Yeah, I'm being pedantic here it's not vital to have this change :) but be nice
if you could, I'm sure Helge probably wouldn't mind too much? :)

>
> > (Being pedantic, they're not quite strictly identical as the
> > ptep_test_and_clear_young() generic function does some weird unnecessary
> > indirection with a local variable and the single {} is dropped etc.)
> >
> > >
> > > Suggested-by: Usama Arif <usama.arif@linux.dev>
> > > Suggested-by: John David Anglin <dave.anglin@bell.net>
> > > Signed-off-by: Helge Deller <deller@gmx.de>
> > > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> >
> > Nits above notwithstanding, LGTM so:
> >
> > Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>
> Thanks!
>
> --
> Pedro

--
Cheers, Lorenzo

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Lorenzo Stoakes (ARM)]

On Tue, Aug 04, 2026 at 01:38:52PM +0100, Pedro Falcato wrote:
> On Tue, Aug 04, 2026 at 12:13:13PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
> > > There is no need for write access to the PTE.
> > >
> > > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> >
> > With below addressed LGTM so:
> >
> > Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> >
> > > ---
> > >  arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > > index e2ea8ba9f8ca..f2bab31040c7 100644
> > > --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > > +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > > @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
> > >
> > >  #ifdef CONFIG_PPC_16K_PAGES
> > >  #define ptep_get ptep_get
> > > -static inline pte_t ptep_get(pte_t *ptep)
> > > +static inline pte_t ptep_get(const pte_t *ptep)
> > >  {
> > >  	pte_basic_t val = READ_ONCE(ptep->pte);
> > >  	pte_t pte = {val, val, val, val};
> >
> > Worth making these const too (that {val, val, val, val} horrifies me btw :)?
>
> This is PPC code, so I don't know if they have any particular opinion here,
> but I could definitely do this.
>
> (FWIW, it seems we're more aggressive in MM in doing this than other places
> in the kernel?)

I mean I can't see why there'd be any objection given you're already const-ing
here :)

>
> --
> Pedro

--
Cheers, Lorenzo

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Christophe Leroy (CS GROUP)]

Hi Lorenzo,

Le 04/08/2026 à 13:13, Lorenzo Stoakes (ARM) a écrit :
> On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
>> There is no need for write access to the PTE.
>>
>> Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> 
> With below addressed LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> 
>> ---
>>   arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>> index e2ea8ba9f8ca..f2bab31040c7 100644
>> --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>> +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>> @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
>>
>>   #ifdef CONFIG_PPC_16K_PAGES
>>   #define ptep_get ptep_get
>> -static inline pte_t ptep_get(pte_t *ptep)
>> +static inline pte_t ptep_get(const pte_t *ptep)
>>   {
>>   	pte_basic_t val = READ_ONCE(ptep->pte);
>>   	pte_t pte = {val, val, val, val};
> 
> Worth making these const too (that {val, val, val, val} horrifies me btw :)?

Any suggestion welcome.

powerpc 8xx page table is independant on page size. When you use 16k 
pages, depending on the address you hit the page for the first time, the 
HW assist page table walk will fetch one of four 4k cells in page table 
that need to be identical as they all four define the same 16k page. Not 
sure I'm clear.

Christophe

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Lorenzo Stoakes (ARM)]

On Tue, Aug 04, 2026 at 02:50:19PM +0200, Christophe Leroy (CS GROUP) wrote:
> Hi Lorenzo,
>
> Le 04/08/2026 à 13:13, Lorenzo Stoakes (ARM) a écrit :
> > On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
> > > There is no need for write access to the PTE.
> > >
> > > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> >
> > With below addressed LGTM so:
> >
> > Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> >
> > > ---
> > >   arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > > index e2ea8ba9f8ca..f2bab31040c7 100644
> > > --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > > +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> > > @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
> > >
> > >   #ifdef CONFIG_PPC_16K_PAGES
> > >   #define ptep_get ptep_get
> > > -static inline pte_t ptep_get(pte_t *ptep)
> > > +static inline pte_t ptep_get(const pte_t *ptep)
> > >   {
> > >   	pte_basic_t val = READ_ONCE(ptep->pte);
> > >   	pte_t pte = {val, val, val, val};
> >
> > Worth making these const too (that {val, val, val, val} horrifies me btw :)?
>
> Any suggestion welcome.

I'm being super nitty, all I mean is:

-   	pte_basic_t val = READ_ONCE(ptep->pte);
-  	pte_t pte = {val, val, val, val};
+   	const pte_basic_t val = READ_ONCE(ptep->pte);
+  	const pte_t pte = {val, val, val, val};

:)

>
> powerpc 8xx page table is independant on page size. When you use 16k pages,
> depending on the address you hit the page for the first time, the HW assist
> page table walk will fetch one of four 4k cells in page table that need to
> be identical as they all four define the same 16k page. Not sure I'm clear.

No that's clear, thanks!

(I say 'horrifying' because I am looking into RCU page table freeing which this
may complicate, though perhaps not in practice, to be continued :)

>
> Christophe
>

--
Cheers, Lorenzo

Re: [PATCH v2 2/6] parisc: Drop own implementations for ptep_get() and ptep_test_and_clear_young()

[Helge Deller]

On 8/4/26 14:42, Lorenzo Stoakes (ARM) wrote:
> On Tue, Aug 04, 2026 at 01:34:41PM +0100, Pedro Falcato wrote:
>> On Tue, Aug 04, 2026 at 12:11:08PM +0100, Lorenzo Stoakes (ARM) wrote:
>>> On Mon, Aug 03, 2026 at 05:43:56PM +0100, Pedro Falcato wrote:
>>>> From: Helge Deller <deller@gmx.de>
>>>>
>>>> Switch to the generic implementations, which are identical.
>>>
>>> You sure do like succinct commit messages :)
>>
>> I didn't even write this one! See the From: :))
> 
> Lol well then feedback addressed to Helge ;)

Noted :-)

>>> Maybe worth saying by dropping the __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG and
>>> ptep_get defines you get the generic versions from include/pgtable.h which are
>>> functionally identical.

Isn't that basically the same as:
"Switch to the generic implementations, which are identical."
;-)

>> I can touch it up though, if you insist.
> 
> Yeah, I'm being pedantic here it's not vital to have this change :) but be nice
> if you could, I'm sure Helge probably wouldn't mind too much? :)

Yes, I'm fine with any cleanup/rephrasing of the commit message.
Thanks!
Helge

>>> (Being pedantic, they're not quite strictly identical as the
>>> ptep_test_and_clear_young() generic function does some weird unnecessary
>>> indirection with a local variable and the single {} is dropped etc.)

Re: [PATCH v2 2/6] parisc: Drop own implementations for ptep_get() and ptep_test_and_clear_young()

[Lorenzo Stoakes (ARM)]

On Tue, Aug 04, 2026 at 03:00:43PM +0200, Helge Deller wrote:
> On 8/4/26 14:42, Lorenzo Stoakes (ARM) wrote:
> > On Tue, Aug 04, 2026 at 01:34:41PM +0100, Pedro Falcato wrote:
> > > On Tue, Aug 04, 2026 at 12:11:08PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > > On Mon, Aug 03, 2026 at 05:43:56PM +0100, Pedro Falcato wrote:
> > > > > From: Helge Deller <deller@gmx.de>
> > > > >
> > > > > Switch to the generic implementations, which are identical.
> > > >
> > > > You sure do like succinct commit messages :)
> > >
> > > I didn't even write this one! See the From: :))
> >
> > Lol well then feedback addressed to Helge ;)
>
> Noted :-)
>
> > > > Maybe worth saying by dropping the __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG and
> > > > ptep_get defines you get the generic versions from include/pgtable.h which are
> > > > functionally identical.
>
> Isn't that basically the same as:
> "Switch to the generic implementations, which are identical."
> ;-)

I am being _exceedingly_, possibly outrageously pedantic here :>)

>
> > > I can touch it up though, if you insist.
> >
> > Yeah, I'm being pedantic here it's not vital to have this change :) but be nice
> > if you could, I'm sure Helge probably wouldn't mind too much? :)
>
> Yes, I'm fine with any cleanup/rephrasing of the commit message.

Thanks :)

> Thanks!
> Helge
>
> > > > (Being pedantic, they're not quite strictly identical as the
> > > > ptep_test_and_clear_young() generic function does some weird unnecessary
> > > > indirection with a local variable and the single {} is dropped etc.)

--
Cheers, Lorenzo

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[LEROY Christophe]

Le 04/08/2026 à 14:59, Lorenzo Stoakes (ARM) a écrit :
> On Tue, Aug 04, 2026 at 02:50:19PM +0200, Christophe Leroy (CS GROUP) wrote:
>> Hi Lorenzo,
>>
>> Le 04/08/2026 à 13:13, Lorenzo Stoakes (ARM) a écrit :
>>> On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
>>>> There is no need for write access to the PTE.
>>>>
>>>> Signed-off-by: Pedro Falcato <pfalcato@suse.de>
>>>
>>> With below addressed LGTM so:
>>>
>>> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>>>
>>>> ---
>>>>    arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>>>> index e2ea8ba9f8ca..f2bab31040c7 100644
>>>> --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>>>> +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>>>> @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
>>>>
>>>>    #ifdef CONFIG_PPC_16K_PAGES
>>>>    #define ptep_get ptep_get
>>>> -static inline pte_t ptep_get(pte_t *ptep)
>>>> +static inline pte_t ptep_get(const pte_t *ptep)
>>>>    {
>>>>    	pte_basic_t val = READ_ONCE(ptep->pte);
>>>>    	pte_t pte = {val, val, val, val};
>>>
>>> Worth making these const too (that {val, val, val, val} horrifies me btw :)?
>>
>> Any suggestion welcome.
> 
> I'm being super nitty, all I mean is:
> 
> -   	pte_basic_t val = READ_ONCE(ptep->pte);
> -  	pte_t pte = {val, val, val, val};
> +   	const pte_basic_t val = READ_ONCE(ptep->pte);
> +  	const pte_t pte = {val, val, val, val};
> 
> :)

I'm fine with that, I was reacting on the "horrifying".

> 
>>
>> powerpc 8xx page table is independant on page size. When you use 16k pages,
>> depending on the address you hit the page for the first time, the HW assist
>> page table walk will fetch one of four 4k cells in page table that need to
>> be identical as they all four define the same 16k page. Not sure I'm clear.
> 
> No that's clear, thanks!
> 
> (I say 'horrifying' because I am looking into RCU page table freeing which this
> may complicate, though perhaps not in practice, to be continued :)

A few more details here if needed: 55c8fc3f4930 ("powerpc/8xx: 
reintroduce 16K pages with HW assistance")

Christophe

Re: [PATCH v2 3/6] mm/powerpc/8xx: constify ptep_get() argument

[Christophe Leroy (CS GROUP)]

Le 04/08/2026 à 14:59, Lorenzo Stoakes (ARM) a écrit :
> On Tue, Aug 04, 2026 at 02:50:19PM +0200, Christophe Leroy (CS GROUP) wrote:
>> Hi Lorenzo,
>>
>> Le 04/08/2026 à 13:13, Lorenzo Stoakes (ARM) a écrit :
>>> On Mon, Aug 03, 2026 at 05:43:57PM +0100, Pedro Falcato wrote:
>>>> There is no need for write access to the PTE.
>>>>
>>>> Signed-off-by: Pedro Falcato <pfalcato@suse.de>
>>>
>>> With below addressed LGTM so:
>>>
>>> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>>>
>>>> ---
>>>>    arch/powerpc/include/asm/nohash/32/pte-8xx.h | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>>>> index e2ea8ba9f8ca..f2bab31040c7 100644
>>>> --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>>>> +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
>>>> @@ -226,7 +226,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
>>>>
>>>>    #ifdef CONFIG_PPC_16K_PAGES
>>>>    #define ptep_get ptep_get
>>>> -static inline pte_t ptep_get(pte_t *ptep)
>>>> +static inline pte_t ptep_get(const pte_t *ptep)
>>>>    {
>>>>    	pte_basic_t val = READ_ONCE(ptep->pte);
>>>>    	pte_t pte = {val, val, val, val};
>>>
>>> Worth making these const too (that {val, val, val, val} horrifies me btw :)?
>>
>> Any suggestion welcome.
> 
> I'm being super nitty, all I mean is:
> 
> -   	pte_basic_t val = READ_ONCE(ptep->pte);
> -  	pte_t pte = {val, val, val, val};
> +   	const pte_basic_t val = READ_ONCE(ptep->pte);
> +  	const pte_t pte = {val, val, val, val};
> 
> :)

I'm fine with that, I was reacting on the "horrifying".

> 
>>
>> powerpc 8xx page table is independant on page size. When you use 16k pages,
>> depending on the address you hit the page for the first time, the HW assist
>> page table walk will fetch one of four 4k cells in page table that need to
>> be identical as they all four define the same 16k page. Not sure I'm clear.
> 
> No that's clear, thanks!
> 
> (I say 'horrifying' because I am looking into RCU page table freeing which this
> may complicate, though perhaps not in practice, to be continued :)

A few more details here if needed: 55c8fc3f4930 ("powerpc/8xx: 
reintroduce 16K pages with HW assistance")

Christophe

Re: [PATCH v2 6/6] mm: constify the pte_offset_map_ro_nolock() return value

[Pedro Falcato]

On Tue, Aug 04, 2026 at 12:22:19PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Aug 03, 2026 at 05:44:00PM +0100, Pedro Falcato wrote:
> > Constify the pte_t * retval from pte_offset_map_ro_nolock(), for which it is
> > already pledged that accesses must be read-only. With it, convert the three
> > treewide users to use const pte_t *.
> >
> > khugepaged passes the result right down to fault code (do_swap_page()). This
> > leads to a complicated set of conditions that, in order to be correct, must
> > not install anything into *vmf->pte. This is not trivial to work around in
> > fault code, and as such just trivially cast to non-const pte_t* in the
> > meantime.
> >
> > The other users are far more trivial and the conversion is equally
> > trivially simple.
> 
> Ah finally more words! :)

:)

> 
> >
> > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> 
> With comment updated as below and nits addressed, LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> 
> > ---
> >  arch/powerpc/mm/pgtable.c | 2 +-
> >  include/linux/mm.h        | 4 ++--
> >  include/linux/pgtable.h   | 2 +-
> >  mm/filemap.c              | 2 +-
> >  mm/khugepaged.c           | 2 +-
> >  mm/pgtable-generic.c      | 4 ++--
> >  6 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
> > index a9be337be3e4..e29db41b6043 100644
> > --- a/arch/powerpc/mm/pgtable.c
> > +++ b/arch/powerpc/mm/pgtable.c
> > @@ -390,7 +390,7 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
> >  	p4d_t *p4d;
> >  	pud_t *pud;
> >  	pmd_t *pmd;
> > -	pte_t *pte;
> > +	const pte_t *pte;
> >  	spinlock_t *ptl;
> >
> >  	if (mm == &init_mm)
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 7fabe6c66b4b..acf5a5e31d34 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3885,8 +3885,8 @@ static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr)
> >  pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> >  			   unsigned long addr, spinlock_t **ptlp);
> >
> > -pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> > -				unsigned long addr, spinlock_t **ptlp);
> > +const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> > +				      unsigned long addr, spinlock_t **ptlp);
> >  pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
> >  				unsigned long addr, pmd_t *pmdvalp,
> >  				spinlock_t **ptlp);
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index dc418553e57a..dd51e722c535 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -112,7 +112,7 @@ static inline pte_t *__pte_map(pmd_t *pmd, unsigned long address)
> >  {
> >  	return pte_offset_kernel(pmd, address);
> >  }
> > -static inline void pte_unmap(pte_t *pte)
> > +static inline void pte_unmap(const pte_t *pte)
> 
> I was going to question this based on whether the contract holds for
> CONFIG_HIGHPTE but actually:
> 
> #define pte_unmap(pte)	do {	\
> 	kunmap_local((pte));	\
> 	rcu_read_unlock();	\
> } while (0)
> 
> #define kunmap_local(__addr)					\
> do {								\
> 	BUILD_BUG_ON(__same_type((__addr), struct page *));	\
> 	__kunmap_local(__addr);					\
> } while (0)
> 
> static inline void __kunmap_local(const void *vaddr) <-- const!
> {
> 	kunmap_local_indexed(vaddr);
> }
> 
> So nice (CONFIG_HIGHPTE is going to go away at some point though, right? I
> hope... :)
> 
> 
> >  {
> >  	rcu_read_unlock();
> >  }
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 6afec636881f..af5d3fcd1b05 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -3490,7 +3490,7 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
> >  {
> >  	struct vm_area_struct *vma = vmf->vma;
> >  	vm_fault_t ret = 0;
> > -	pte_t *ptep;
> > +	const pte_t *ptep;
> >
> >  	/*
> >  	 * We might have COW'ed a pagecache folio and might now have an mlocked
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b237f6e7662a..09efac93a8c6 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1170,7 +1170,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
> >  			 * Here the ptl is only used to check pte_same() in
> >  			 * do_swap_page(), so readonly version is enough.
> >  			 */
> > -			pte = pte_offset_map_ro_nolock(mm, pmd, addr, &ptl);
> > +			pte = (pte_t *) pte_offset_map_ro_nolock(mm, pmd, addr, &ptl);
> 
> Hmm yeah this is nasty, but you explain why in the commit message. Could you
> extend the comment to explain it?

Yep, I'll do it.

> 
> >  			if (!pte) {
> >  				mmap_read_unlock(mm);
> >  				result = SCAN_NO_PTE_TABLE;
> > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> > index b91b1a98029c..2cfc6e608ef4 100644
> > --- a/mm/pgtable-generic.c
> > +++ b/mm/pgtable-generic.c
> > @@ -308,8 +308,8 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
> >  	return NULL;
> >  }
> >
> > -pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> > -				unsigned long addr, spinlock_t **ptlp)
> > +const pte_t *pte_offset_map_ro_nolock(struct mm_struct *mm, pmd_t *pmd,
> 
> Can pmd be const too?

I was going to say "yes but then pmdp_get() also needs to be constfified" but
actually no, it can't:

pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp) {
        if (unlikely(pmd_bad(pmdval))) {
                pmd_clear_bad(pmd);
                goto nomap;
        }
}

so PTE mapping actually needs to write to the pmdp if the pmd looks bad.
Tricky stuff :)

-- 
Pedro