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

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Message-ID: <56536114.1020305@android.com>
Date:	Mon, 23 Nov 2015 10:55:16 -0800
From:	Daniel Cashman <dcashman@...roid.com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-kernel@...r.kernel.org, linux@....linux.org.uk,
	akpm@...ux-foundation.org, keescook@...omium.org, mingo@...nel.org,
	linux-arm-kernel@...ts.infradead.org, corbet@....net,
	dzickus@...hat.com, ebiederm@...ssion.com, xypron.glpk@....de,
	jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
	n-horiguchi@...jp.nec.com, aarcange@...hat.com, mgorman@...e.de,
	tglx@...utronix.de, rientjes@...gle.com, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, salyzyn@...roid.com, jeffv@...gle.com,
	nnk@...gle.com, catalin.marinas@....com, hpa@...or.com,
	x86@...nel.org, hecmargi@....es, bp@...e.de, dcashman@...gle.com
Subject: Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

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
>> +       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.

We could move this logic into the code where min/max are initialized,
but that would create its own mess, creating new Kconfig values to
introduce it in an arch-agnostic way after patch-set v2 moved that to
mm/mmap.c instread of arch/${arch}/mm/mmap.c Suggestions welcome.

Thank You,
Dan

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