[PATCH v4 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 (mm) 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).

This series adds const qualifiers to pointer parameters in functions that
perform read-only operations on:
- struct page, folio, and ptdesc
- struct vm_area_struct and vm_fault
- struct mm_struct and address_space
- struct zone, lruvec, and mem_section
- Various architecture-specific mm structures

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)

Max Kellermann (12):
  mm/shmem: add `const` to pointer parameters for improved
    const-correctness
  mm/pagemap: add `const` to pointer parameters for improved
    const-correctness
  mm/mmzone: add const to pointer parameters for improved
    const-correctness
  fs: add const to pointer parameters for improved const-correctness
  mm/oom_kill: add const to pointer parameter for improved
    const-correctness
  mm/util, s390: add const to pointer parameters for improved
    const-correctness
  parisc: add `const` to mmap_upper_limit() parameter
  mm/util, s390, sparc, x86: add const to arch_pick_mmap_layout()
    parameter
  mm/mm_types: add const to pointer parameters for improved
    const-correctness
  mm/mm_inline: add const to pointer parameters for improved
    const-correctness
  mm: add const to pointer parameters for improved const-correctness
  mm/highmem: add const to pointer parameters 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    | 38 ++++++++++---------
 include/linux/highmem.h             |  8 ++--
 include/linux/mm.h                  | 48 +++++++++++------------
 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                       |  3 +-
 mm/shmem.c                          |  6 +--
 mm/util.c                           | 20 ++++++----
 21 files changed, 162 insertions(+), 146 deletions(-)

-- 
2.47.2