linux-kernel - Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND

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Date:	Fri, 27 Nov 2015 11:36:30 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Daniel Cashman <dcashman@...roid.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, dcashman@...gle.com,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"H. Peter Anvin" <hpa@...or.com>, mingo <mingo@...nel.org>,
	aarcange@...hat.com, Russell King <linux@....linux.org.uk>,
	Jonathan Corbet <corbet@....net>, xypron.glpk@....de,
	"x86@...nel.org" <x86@...nel.org>, hecmargi@....es,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Borislav Petkov <bp@...e.de>, nnk@...gle.com,
	dzickus@...hat.com, Kees Cook <keescook@...omium.org>,
	jpoimboe@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-arm-kernel@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>, salyzyn@...roid.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>, jeffv@...gle.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

2015-11-25 23:39 GMT+03:00 Daniel Cashman <dcashman@...roid.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
>

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?

> 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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