[PATCH v2] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

[PATCH v2] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

[Kornel Dulęba]

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>
---

v2:
- Update commit message to explain why we need to set the same value as
  in the 48-bit case.

 arch/arm64/Kconfig | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a182295e6f08..6527d0d5656a 100644
--- 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
-- 
2.49.0.777.g153de2bbd5-goog

Re: [PATCH v2] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

[Will Deacon]

On Thu, 17 Apr 2025 11:47:54 +0000, Kornel Dulęba wrote:
> 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.
> 
> [...]

Applied to arm64 (for-next/mm), thanks!

[1/1] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX
      https://git.kernel.org/arm64/c/f101c5644771

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
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