Date: Fri, 27 Nov 2015 09:32:50 +0000
From: Catalin Marinas <catalin.marinas@arm.com>
To: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Daniel Cashman <dcashman@android.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Will Deacon <will.deacon@arm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "H. Peter Anvin" <hpa@zytor.com>, mingo <mingo@kernel.org>,
        aarcange@redhat.com, Russell King <linux@arm.linux.org.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>, xypron.glpk@gmx.de,
        "x86@kernel.org" <x86@kernel.org>, hecmargi@upv.es,
        Mel Gorman <mgorman@suse.de>, David Rientjes <rientjes@google.com>,
        Borislav Petkov <bp@suse.de>, nnk@google.com, dzickus@redhat.com,
        Kees Cook <keescook@chromium.org>, jpoimboe@redhat.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>, salyzyn@android.com,
        "Eric W. Biederman" <ebiederm@xmission.com>, jeffv@google.com,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, dcashman@google.com
Subject: Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.
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On Fri, Nov 27, 2015 at 11:36:30AM +0300, Andrey Ryabinin wrote:
> 2015-11-25 23:39 GMT+03:00 Daniel Cashman <dcashman@android.com>:
> > On 11/25/2015 04:06 AM, Catalin Marinas wrote:
> >> 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?
> >
> > The general formula I used ended up being:
> > _max = floor(log(TASK_SIZE)) - log(PAGE_SIZE) - 3
> 
> For kasan, we calculate KASAN_SHADOW_OFFSET in Makefile, because we need to use
> that value in Makefiles.
> 
> For ARCH_MMAP_RND_COMPAT_BITS_MIN/MAX I don't see a reason why it has
> to be in Kconfig.
> Can't we just use your formula to #define ARCH_MMAP_RND_COMPAT_BITS_*
> in some arch header?

Because there is another option, ARCH_MMAP_RND_BITS depending on EXPERT
which uses the MIN/MAX range defined per architecture. Since it's an
expert feature, we could as well ignore the MIN/MAX in Kconfig and just
add BUILD_BUG_ON checks to the code. This way we could simply define
them in C files.

Alternatively, add arithmetics support to kbuild ;).

-- 
Catalin
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