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

[Daniel Cashman <dcashman@android.com>]

On 11/25/2015 04:06 AM, Catalin Marinas wrote:
> 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.

It is not, and should have been ARM64_VA_BITS.  This stanza was meant to
mimic the one for ARM64_VA_BITS.  Thank you for pointing this, and the
39-bit combination out.

>>>> +       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?

The general formula I used ended up being:
_max = floor(log(TASK_SIZE)) - log(PAGE_SIZE) - 3

which in the case of arm64 ended up being VA_BITS - PAGE_SHIFT - 3.
Aside: following this would actually put COMPAT_BITS_MAX at 17 for 4k
pages, rather than 16, but I left it at 16 to mirror what was put in
arch/arm/Kconfig.


Thank You,
Dan

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