[PATCH v5 00/12] mm: establish const-correctness for pointer parameters

[Max Kellermann <max.kellermann@ionos.com>]

For improved const-correctness.

This patch series systematically adds const qualifiers to pointer
parameters throughout the memory management subsystem, establishing a
foundation for improved const-correctness across the entire Linux
kernel.

Const-correctness provides multiple benefits:

1. Type Safety: The compiler enforces that functions marked as taking
   const parameters cannot accidentally modify the data, catching
   potential bugs at compile time rather than runtime.

2. Compiler Optimizations: When the compiler knows data won't be
   modified, it can generate more efficient code through better
   register allocation, code motion, and aliasing analysis.

3. API Documentation: Const qualifiers serve as self-documenting code,
   making it immediately clear to developers which functions are
   read-only operations versus those that modify state.

4. Maintenance Safety: Future modifications to const-correct code are
   less likely to introduce subtle bugs, as the compiler will reject
   attempts to modify data that should remain unchanged.

The memory management subsystem is a fundamental building block of the
kernel.  Most higher-level kernel subsystems (filesystems, drivers,
networking) depend on mm interfaces.  By establishing
const-correctness at this foundational level:

1. Enables Propagation: Higher-level subsystems can adopt
   const-correctness in their own interfaces.  Without const-correct
   mm functions, filesystems cannot mark their own parameters const
   when they need to call mm functions.

2. Maximum Impact: Changes to core mm APIs benefit the entire kernel, as
   these functions are called from virtually every subsystem.

3. Prevents Impedance Mismatch: Without const-correctness in mm, other
   subsystems must either cast away const (dangerous) or avoid using
   const altogether (missing optimization opportunities).

Each patch focuses on a specific header or subsystem component to ease review
and bisection.

This work was initially posted as a single large patch:
 https://lore.kernel.org/lkml/20250827192233.447920-1-max.kellermann@ionos.com/

Following feedback from Lorenzo Stoakes and David Hildenbrand, it has been
split into focused, reviewable chunks. The approach was validated with a
smaller patch that received agreement:
 https://lore.kernel.org/lkml/20250828130311.772993-1-max.kellermann@ionos.com/

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
v1 -> v2:
- made several parameter values const (i.e. the pointer address, not
  just the pointed-to memory), as suggested by Andrew Morton and
  Yuanchu Xie
- drop existing+obsolete "extern" keywords on lines modified by these
  patches (suggested by Vishal Moola)
- add missing parameter names on lines modified by these patches
  (suggested by Vishal Moola)
- more "const" pointers (e.g. the task_struct passed to
  process_shares_mm())
- add missing "const" to s390, fixing s390 build failure
- moved the mmap_is_legacy() change in arch/s390/mm/mmap.c from 08/12
  to 06/12 (suggested by Vishal Moola)

v2 -> v3:
- remove garbage from 06/12
- changed tags on subject line (suggested by Matthew Wilcox)

v3 -> v4:
- more verbose commit messages including a listing of function names
  (suggested by David Hildenbrand and Lorenzo Stoakes)

v4 -> v5:
- back to shorter commit messages after an agreement between David
  Hildenbrand and Lorenzo Stoakes was found

Max Kellermann (12):
  mm: constify shmem related test functions for improved
    const-correctness
  mm: constify pagemap related test functions for improved
    const-correctness
  mm: constify zone related test functions for improved
    const-correctness
  fs: constify mapping related test functions for improved
    const-correctness
  mm: constify process_shares_mm() for improved const-correctness
  mm, s390: constify mapping related test functions for improved
    const-correctness
  parisc: constify mmap_upper_limit() parameter for improved
    const-correctness
  mm: constify arch_pick_mmap_layout() for improved const-correctness
  mm: constify ptdesc_pmd_pts_count() and folio_get_private()
  mm: constify various inline test functions for improved
    const-correctness
  mm: constify assert/test functions in mm.h
  mm: constify highmem related functions for improved const-correctness

 arch/arm/include/asm/highmem.h      |  6 +--
 arch/parisc/include/asm/processor.h |  2 +-
 arch/parisc/kernel/sys_parisc.c     |  2 +-
 arch/s390/mm/mmap.c                 |  7 ++--
 arch/sparc/kernel/sys_sparc_64.c    |  3 +-
 arch/x86/mm/mmap.c                  |  7 ++--
 arch/xtensa/include/asm/highmem.h   |  2 +-
 include/linux/fs.h                  |  7 ++--
 include/linux/highmem-internal.h    | 44 +++++++++++----------
 include/linux/highmem.h             |  8 ++--
 include/linux/mm.h                  | 56 +++++++++++++--------------
 include/linux/mm_inline.h           | 26 +++++++------
 include/linux/mm_types.h            |  4 +-
 include/linux/mmzone.h              | 42 ++++++++++----------
 include/linux/pagemap.h             | 59 +++++++++++++++--------------
 include/linux/sched/mm.h            |  4 +-
 include/linux/shmem_fs.h            |  4 +-
 mm/highmem.c                        | 10 ++---
 mm/oom_kill.c                       |  7 ++--
 mm/shmem.c                          |  6 +--
 mm/util.c                           | 20 ++++++----
 21 files changed, 171 insertions(+), 155 deletions(-)

-- 
2.47.2