linux-kernel - Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64

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Date:	Mon, 19 Mar 2007 14:27:04 +0000
From:	Andy Whitcroft <apw@...dowen.org>
To:	Martin Bligh <mbligh@...igh.org>
CC:	Christoph Lameter <clameter@....com>,
	Andi Kleen <andi@...stfloor.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Glauber de Oliveira Costa <glommer@...il.com>
Subject: Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64 -
 Take 2

Martin Bligh wrote:
> Christoph Lameter wrote:
>> On Fri, 16 Mar 2007, Martin Bligh wrote:
>>
>>> You have to do some sort of lookup anyway, and Andy seemed to have them
>>> all folded into one.
>>
>> What lookup would you need to do? On x86_64 even the TLB use is hidden
>> by the existing 2M entries for 1-1 mappings.
>>
>>> Or are you trying to avoid this by going to back to the crud we had
>>> in 2.4 where we pretend mem_map is one big array, indexed by pfn with
>>> huge sparsely mapped holes in it?
>>
>> Yes that the advanced way of doing it rather than adding useless
>> custom lookups.
> 
> For starters, you can't do that sparse a mapping on a 32 bit system.
> I'll let Andy explain the rest of it.
> 
>>> Would be nice to work out (and document somewhere) what the pros and
>>> cons of virtual memmap vs sparsemem were - ISTR one of the arguments
>>> was extremely sparsely layed out machines, and you needed sparsemem
>>> for that. But right now we have 3 solutions, which is not a good
>>> situation.
>>
>> Please read my posts to linux-mm on that subject. We discussed it last
>> year in detail and the agreement was that the sparsemem crud needs to
>> be taken out. Kame-san posted patches to do that.
> 
> "the agreement"? So Andy agreed to taking it out? Or you and Kame did?

The discussions centred around some patches from Kame which introduced a
SPARSMEM sub-model with a virtual memory map.  That was a supprisingly
clean change which if followed through to its logical conclusion would
remove a significant chunk of architecture specific vmemmap
implementation from ia64, and (as I understand it) was likely to allow
the same to be reused in s390x as well.

SPARSEMEM would still have its useful modes for smaller memory systems.

-apw
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