Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

[Catalin Marinas]

On Fri, Nov 27, 2015 at 11:36:30AM +0300, Andrey Ryabinin wrote:
> 2015-11-25 23:39 GMT+03:00 Daniel Cashman <dcashman@xxxxxxxxxxx>:
> > 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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