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

[Pedro Falcato <pfalcato@suse.de>]

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