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

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Message-ID: <CAKgNAkhn+kTAq6_VKB3GjwwZGD-YOKE67=4fp+SR=1Lbhz7Bxg@mail.gmail.com>
Date:	Mon, 22 Oct 2012 18:42:22 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Andi Kleen <andi@...stfloor.org>, 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

On Mon, Oct 22, 2012 at 6:29 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> 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.)
>
> Some architectures have configurable huge page sizes, so it depends on
> the user. I thought 16K is reasonable.  Can make it larger too.
>
> But I personally consider even 16GB pages somewhat too big.

I do not know the answer course ;-). Just thought that it was worth
emphasizing that some system already allows the upper limit you
propose. It seems inevitable that some other system will allow
something even bigger.

Anyway, I got distracted from my earlier more important point. This
proposed change will chew up most (all?) of the remaining bit-space in
'flags'. This seems like a mistake from a future extensibility point
of view... It sounds a lot like you'll force someone else to write and
deploy mmap3()...

-- 
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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