Date: Wed, 25 Nov 2015 12:06:01 +0000
From: Catalin Marinas <catalin.marinas@arm.com>
To: Daniel Cashman <dcashman@android.com>
Cc: Will Deacon <will.deacon@arm.com>, dcashman@google.com,
        linux-doc@vger.kernel.org, linux-mm@kvack.org, hpa@zytor.com,
        mingo@kernel.org, aarcange@redhat.com, linux@arm.linux.org.uk,
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        salyzyn@android.com, ebiederm@xmission.com, jeffv@google.com,
        akpm@linux-foundation.org, kirill.shutemov@linux.intel.com
Subject: Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.
Message-ID: <20151125120601.GC3109@e104818-lin.cambridge.arm.com>
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On Mon, Nov 23, 2015 at 10:55:16AM -0800, Daniel Cashman wrote:
> On 11/23/2015 07:04 AM, Will Deacon wrote:
> > On Wed, Nov 18, 2015 at 03:20:07PM -0800, Daniel Cashman wrote:
> >> +config ARCH_MMAP_RND_BITS_MAX
> >> +       default 20 if ARM64_64K_PAGES && ARCH_VA_BITS=39

Where is ARCH_VA_BITS defined? We only have options like
ARM64_VA_BITS_39.

BTW, we no longer allow the 64K pages and 39-bit VA combination.

> >> +       default 24 if ARCH_VA_BITS=39
> >> +       default 23 if ARM64_64K_PAGES && ARCH_VA_BITS=42
> >> +       default 27 if ARCH_VA_BITS=42
> >> +       default 29 if ARM64_64K_PAGES && ARCH_VA_BITS=48
> >> +       default 33 if ARCH_VA_BITS=48
> >> +       default 15 if ARM64_64K_PAGES
> >> +       default 19
> >> +
> >> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> >> +       default 7 if ARM64_64K_PAGES
> >> +       default 11
> > 
> > FYI: we now support 16k pages too, so this might need updating. It would
> > be much nicer if this was somehow computed rather than have the results
> > all open-coded like this.
> 
> Yes, I ideally wanted this to be calculated based on the different page
> options and VA_BITS (which itself has a similar stanza), but I don't
> know how to do that/if it is currently supported in Kconfig. This would
> be even more desirable with the addition of 16K_PAGES, as with this
> setup we have a combinatorial problem.

For KASan, we ended up calculating KASAN_SHADOW_OFFSET in
arch/arm64/Makefile. What would the formula be for the above
ARCH_MMAP_RND_BITS_MAX?

-- 
Catalin
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