[PATCH v4 08/12] mm/util, s390, sparc, x86: add const to arch_pick_mmap_layout() parameter

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

The memory management (mm) subsystem is a fundamental low-level component
of the Linux kernel. Establishing const-correctness at this foundational
level enables higher-level subsystems, such as filesystems and drivers,
to also adopt const-correctness in their interfaces. This patch lays
the groundwork for broader const-correctness throughout the kernel
by starting with the core mm subsystem.

This patch adds const qualifiers to mm_struct and rlimit pointer
parameters in arch_pick_mmap_layout() across multiple architectures,
improving type safety and enabling compiler optimizations.

Functions improved:
- arch_pick_mmap_layout() (s390, sparc, x86, and generic mm/util.c)
- mmap_base() (x86)

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 arch/s390/mm/mmap.c              | 5 +++--
 arch/sparc/kernel/sys_sparc_64.c | 3 ++-
 arch/x86/mm/mmap.c               | 7 ++++---
 include/linux/sched/mm.h         | 4 ++--
 mm/util.c                        | 9 ++++++---
 5 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/arch/s390/mm/mmap.c b/arch/s390/mm/mmap.c
index c0f619fb9ab3..a47c7133d82a 100644
--- a/arch/s390/mm/mmap.c
+++ b/arch/s390/mm/mmap.c
@@ -47,7 +47,7 @@ static unsigned long mmap_base_legacy(unsigned long rnd)
 }
 
 static inline unsigned long mmap_base(unsigned long rnd,
-				      struct rlimit *rlim_stack)
+				      const struct rlimit *const rlim_stack)
 {
 	unsigned long gap = rlim_stack->rlim_cur;
 	unsigned long pad = stack_maxrandom_size() + stack_guard_gap;
@@ -169,7 +169,8 @@ unsigned long arch_get_unmapped_area_topdown(struct file *filp, unsigned long ad
  * This function, called very early during the creation of a new
  * process VM image, sets up which VM layout function to use:
  */
-void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
+void arch_pick_mmap_layout(struct mm_struct *const mm,
+			   const struct rlimit *const rlim_stack)
 {
 	unsigned long random_factor = 0UL;
 
diff --git a/arch/sparc/kernel/sys_sparc_64.c b/arch/sparc/kernel/sys_sparc_64.c
index 785e9909340f..1c243bb461b2 100644
--- a/arch/sparc/kernel/sys_sparc_64.c
+++ b/arch/sparc/kernel/sys_sparc_64.c
@@ -294,7 +294,8 @@ static unsigned long mmap_rnd(void)
 	return rnd << PAGE_SHIFT;
 }
 
-void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
+void arch_pick_mmap_layout(struct mm_struct *const mm,
+			   const struct rlimit *const rlim_stack)
 {
 	unsigned long random_factor = mmap_rnd();
 	unsigned long gap;
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 708f85dc9380..13bde4ba2f24 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -80,7 +80,7 @@ unsigned long arch_mmap_rnd(void)
 }
 
 static unsigned long mmap_base(unsigned long rnd, unsigned long task_size,
-			       struct rlimit *rlim_stack)
+			       const struct rlimit *const rlim_stack)
 {
 	unsigned long gap = rlim_stack->rlim_cur;
 	unsigned long pad = stack_maxrandom_size(task_size) + stack_guard_gap;
@@ -110,7 +110,7 @@ static unsigned long mmap_legacy_base(unsigned long rnd,
  */
 static void arch_pick_mmap_base(unsigned long *base, unsigned long *legacy_base,
 		unsigned long random_factor, unsigned long task_size,
-		struct rlimit *rlim_stack)
+		const struct rlimit *const rlim_stack)
 {
 	*legacy_base = mmap_legacy_base(random_factor, task_size);
 	if (mmap_is_legacy())
@@ -119,7 +119,8 @@ static void arch_pick_mmap_base(unsigned long *base, unsigned long *legacy_base,
 		*base = mmap_base(random_factor, task_size, rlim_stack);
 }
 
-void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
+void arch_pick_mmap_layout(struct mm_struct *const mm,
+			   const struct rlimit *const rlim_stack)
 {
 	if (mmap_is_legacy())
 		mm_flags_clear(MMF_TOPDOWN, mm);
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2201da0afecc..0232d983b715 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -178,7 +178,7 @@ static inline void mm_update_next_owner(struct mm_struct *mm)
 #endif
 
 extern void arch_pick_mmap_layout(struct mm_struct *mm,
-				  struct rlimit *rlim_stack);
+				  const struct rlimit *rlim_stack);
 
 unsigned long
 arch_get_unmapped_area(struct file *filp, unsigned long addr,
@@ -211,7 +211,7 @@ generic_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
 				  unsigned long flags, vm_flags_t vm_flags);
 #else
 static inline void arch_pick_mmap_layout(struct mm_struct *mm,
-					 struct rlimit *rlim_stack) {}
+					 const struct rlimit *rlim_stack) {}
 #endif
 
 static inline bool in_vfork(struct task_struct *tsk)
diff --git a/mm/util.c b/mm/util.c
index f5a35efba7bf..24f08217cd95 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -431,7 +431,8 @@ static int mmap_is_legacy(const struct rlimit *const rlim_stack)
 #define MIN_GAP		(SZ_128M)
 #define MAX_GAP		(STACK_TOP / 6 * 5)
 
-static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
+static unsigned long mmap_base(const unsigned long rnd,
+			       const struct rlimit *const rlim_stack)
 {
 #ifdef CONFIG_STACK_GROWSUP
 	/*
@@ -462,7 +463,8 @@ static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
 #endif
 }
 
-void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
+void arch_pick_mmap_layout(struct mm_struct *const mm,
+			   const struct rlimit *const rlim_stack)
 {
 	unsigned long random_factor = 0UL;
 
@@ -478,7 +480,8 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 	}
 }
 #elif defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
-void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
+void arch_pick_mmap_layout(struct mm_struct *const mm,
+			   const struct rlimit *const rlim_stack)
 {
 	mm->mmap_base = TASK_UNMAPPED_BASE;
 	mm_flags_clear(MMF_TOPDOWN, mm);
-- 
2.47.2