mm: Change mmap_rnd_bits_max to __ro_after_init

Allow mmap_rnd_bits_max to be updated on architectures that
determine virtual address space size at runtime instead of relying
on Kconfig options by changing it from const to __ro_after_init.
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230929211155.3910949-5-samitolvanen@google.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

include/linux/mm.h

@@ -86,7 +86,7 @@ extern int sysctl_legacy_va_layout;
 
 #ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
 extern const int mmap_rnd_bits_min;
-extern const int mmap_rnd_bits_max;
+extern int mmap_rnd_bits_max __ro_after_init;
 extern int mmap_rnd_bits __read_mostly;
 #endif
 #ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS

mm/mmap.c

@@ -64,7 +64,7 @@
 
 #ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
 const int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
-const int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
+int mmap_rnd_bits_max __ro_after_init = CONFIG_ARCH_MMAP_RND_BITS_MAX;
 int mmap_rnd_bits __read_mostly = CONFIG_ARCH_MMAP_RND_BITS;
 #endif
 #ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS