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

Catalin Marinas
2015-11-25T12:06:11Z

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