linux-kernel - Re: [PATCH] MM: Support more pagesizes for MAP_HUGETLB/SHM

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Date:	Mon, 22 Oct 2012 18:23:32 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH] MM: Support more pagesizes for MAP_HUGETLB/SHM_HUGETLB v6

>> >> But there seems an obvious solution here: given your value in those
>> >> bits (call it 'n'), the why not apply a multiplier. I mean, certainly
>> >> you never want a value <= 12 for n, and I suspect that the reasonable
>> >> minimum could be much larger (e.g., 2^16). Call that minimum M. Then
>> >> you could interpret the value in your bits as meaning a page size of
>> >>
>> >>     (2^n) * M
>> >
>> > I considered that, but it would seem ugly and does not add that
>> > many bits.
>> >
>> >>
>> >> > So this will use up all remaining flag bits now.
>> >>
>> >> On the other hand, that seems really bad. It looks like that kills the
>> >> ability to further extend the mmap() API with new flags in the future.
>> >> It doesn't sound like we should be doing that.
>> >
>> > You can always add flags to PROT or add a mmap3(). Has been done before.
>> > Or just don't do any new MAP_SECURITY_HOLEs
>>
>> There seems to be a reasonable argument here for an mmap3() with a
>> 64-bit flags argument...
>
> It's just a pain to deploy.
>
> I think I would rather do the offset then. That could still handle
> PowerPC
>
> 14 + 31 = 44 = 16GB           (minimum size 16K)

Since PowerPC already allows 16GB page sizes, doesn't there need to be
allowance for the possibility of future expansion? Choosing a larger
minimum size (like 2^16) would allow that. Does the minimum size need
to be 16k? (Surely, if you want a HUGEPAGE, you want a bigger page
than that? I am not sure.)

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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