arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

When the 52-bit virtual addressing was introduced the select like
ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
Because of that the rnd max bits knob is set to the default value of 18
when ARM64_VA_BITS=52.
Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
be used if 48-bit addressing was used. Higher values can't used here
because 52-bit addressing is used only if the caller provides a hint to
mmap, with a fallback to 48-bit. The knob in question is an upper bound
for what the user can set in /proc/sys/vm/mmap_rnd_bits, which in turn
is used to determine how many random bits can be inserted into the base
address used for mmap allocations. Since 48-bit allocations are legal
with ARM64_VA_BITS=52, we need to make sure that the base address is
small enough to facilitate this.

Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
Signed-off-by: Kornel Dulęba <korneld@google.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20250417114754.3238273-1-korneld@google.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a182295e6f08bfa0f3e6f630dc4adfe797a4d273..6527d0d5656a13ca92510c06d8adacced7a99e03 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -333,9 +333,9 @@ config ARCH_MMAP_RND_BITS_MAX
        default 24 if ARM64_VA_BITS=39
        default 27 if ARM64_VA_BITS=42
        default 30 if ARM64_VA_BITS=47
-       default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
-       default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
-       default 33 if ARM64_VA_BITS=48
+       default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
+       default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
+       default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
        default 14 if ARM64_64K_PAGES
        default 16 if ARM64_16K_PAGES
        default 18