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

On Mon, Aug 03, 2026 at 05:43:54PM +0100, 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.

Even though the pte accesses should always be type-safe, this series does
not really address the problem completely and instead changes things only
for a small set of pte access sites. So just wondering how much beneficial
this series really is ?

> 
> [CC list editorially trimmed for brevity reasons; apologies if you're not on it]
> 
> Link: https://lore.kernel.org/linux-mm/[email protected]/#t [0]
> Cc: Catalin Marinas <[email protected]>
> Cc: Will Deacon <[email protected]>
> Cc: "James E.J. Bottomley" <[email protected]>
> Cc: Helge Deller <[email protected]>
> Cc: Madhavan Srinivasan <[email protected]>
> Cc: Michael Ellerman <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: "Liam R. Howlett" <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: "Matthew Wilcox (Oracle)" <[email protected]>
> Cc: Jan Kara <[email protected]>
> Cc: Zi Yan <[email protected]>
> Cc: Baolin Wang <[email protected]>
> Cc: Nico Pache <[email protected]>
> Cc: Ryan Roberts <[email protected]>
> Cc: Dev Jain <[email protected]>
> Cc: Barry Song <[email protected]>
> Cc: Lance Yang <[email protected]>
> Cc: Usama Arif <[email protected]>
> Cc: Kevin Brodsky <[email protected]>
> Cc: Muhammad Usama Anjum <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> 
> 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
>