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Date: Wed, 25 Nov 2015 15:26:41 +1100 From: Michael Ellerman <mpe@...erman.id.au> To: Daniel Cashman <dcashman@...roid.com>, 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 Mon, 2015-11-23 at 10:55 -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 > > > + 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 instead of arch/${arch}/mm/mmap.c Suggestions welcome. Could we instead change the meaning of the mmap_rnd_bits value to be the number of address space bits that may be randomised? ie. 40 would mean "please randomise in a 1T range", which with PAGE_SIZE=4K gives you 28 random bits. etc. That would make the value independent of PAGE_SIZE, and only depend on the size of the address space. It would also mean the values userspace sets and sees don't need to change if the kernel PAGE_SIZE changes. (which probably doesn't happen often but still) cheers -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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