linux-kernel - Re: Arch specific mmap attributes (Was: mprotect pgprot handling weirdness)

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Message-Id: <20100407095145.FB70.A69D9226@jp.fujitsu.com>
Date:	Wed,  7 Apr 2010 15:03:45 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org
Subject: Re: Arch specific mmap attributes (Was: mprotect pgprot handling weirdness)

> On Tue, 2010-04-06 at 19:26 +0900, KOSAKI Motohiro wrote:
> > > Ok, I see. No biggie. The main deal remains how we want to do that
> > > inside the kernel :-) I think the less horrible options here are
> > > to either extend vm_flags to always be 64-bit, or add a separate
> > > vm_map_attributes flag, and add the necessary bits and pieces to
> > > prevent merge accross different attribute vma's.
> > 
> > vma->vm_flags already have VM_SAO. Why do we need more flags?
> > At least, I dislike to add separate flags member into vma.
> > It might introduce unnecessary messy into vma merge thing.
> 
> Well, we did shove SAO in there, and used up the very last vm_flag for
> it a while back. Now I need another one, for little endian mappings. So
> I'm stuck.
> 
> But the problem goes further I believe. Archs do nowadays have quite an
> interesting set of MMU attributes that it would be useful to expose to
> some extent.

Generally speaking, It seems no good idea. desktop and server world don't
interest arch specific mmu attribute crap. because many many opensource
and ISV library don't care it. I know highend hpc and embedded have 
differenct eco-system. they might want to use such strange mmu feature.
I recommend to you are focusing popwerpc eco-system. 

I'm not against changing kernel internal. I only disagree mmu attribute
fashion will be become used widely.


> 
> Some powerpc's also provide storage keys for example and I think ARM
> have something along those lines. There's interesting cachability
> attributes too, on x86 as well. Being able to use such attributes to
> request for example a relaxed ordering mapping on x86 might be useful.
> 
> I think it basically boils down to either extend vm_flags to always be
> 64-bit, which seems to be Nick preferred approach, or introduct a
> vm_attributes with all the necessary changes to the merge code to take
> it into account (not -that- hard tho, there's only half a page of
> results in grep for these things :-)




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