linux-kernel - Re: [PATCH v2] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS

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Message-Id: <174593987866.472512.6873137326879401537.b4-ty@kernel.org>
Date: Tue, 29 Apr 2025 21:27:48 +0100
From: Will Deacon <will@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>,
	Kornel Dulęba <korneld@...gle.com>
Cc: kernel-team@...roid.com,
	Will Deacon <will@...nel.org>,
	Steve Capper <steve.capper@....com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	ssradjacoumar@...gle.com,
	chromeos-krk-upstreaming@...gle.com,
	Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

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
https://will.arm64.dev