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

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Message-ID: <20121022161151.GS16230@one.firstfloor.org>
Date:	Mon, 22 Oct 2012 18:11:51 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Andi Kleen <ak@...ux.intel.com>, 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 05:53:45PM +0200, Michael Kerrisk (man-pages) wrote:
> I'm not sure whether anything is using the high 8 bits of prot, bun
> passing I note that there seems to be no check that the unused bits
> are zeroed so there's a small chance  existing apps are passing random
> garbage there. (Of course, mmap() is hardly the only API to have that
> fault, and it hasn't stopped us from reusing bits in those APIs,
> though sometimes we've gotten bitten by apps that did pass in random
> garbage).

Ok.

> 
> >> 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)

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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