linux-kernel - Re: why choose 896MB to the start point of ZONE

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Message-ID: <m2me6bf505b1004070510q63926780uac2807ca0ab22db6@mail.gmail.com>
Date:	Wed, 7 Apr 2010 07:10:02 -0500
From:	Xianghua Xiao <xiaoxianghua@...il.com>
To:	Venkatram Tummala <venkatram867@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Youngwhan Song <breadncup@...il.com>,
	Joel Fernandes <agnel.joel@...il.com>,
	Frank Hu <frank.hu.2001@...il.com>,
	hayfeng Lee <teklife.kernel@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kernel@...kernel.org" <linux-kernel@...kernel.org>,
	"kernelnewbies@...linux.org" <kernelnewbies@...linux.org>
Subject: Re: why choose 896MB to the start point of ZONE_HIGHMEM

On Wed, Apr 7, 2010 at 12:48 AM, Venkatram Tummala
<venkatram867@...il.com> wrote:
> I completely agree with you. I was just trying to clarify Xianghua's
> statement "last 128 MB is used for HIGHMEM". I got the feeling that he
> thought that last 128MB can be used for vmalloc, IO and for HIGHMEM. So, i
> was clarifying that last 128MB is not "used for highmem" but it is used to
> support highmem.(among many other things). That was what i intended.
>
> On Tue, Apr 6, 2010 at 7:09 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> On 04/06/2010 07:04 PM, Venkatram Tummala wrote:
>> > Hey Xiao,
>> >
>> > last 128MB is not used for highmem. last 128MB is used for data
>> > structures(page tables etc.) to support highmem .  Highmem is not
>> > something which is "INSIDE" Kernel's Virtual Address space. Highmem
>> > refers to a region of "Physical memory" which can be mapped into
>> > kernel's virtual address space through page tables.
>> >
>> > Regards,
>> > Venkatram Tummala
>> >
>>
>> Not quite.
>>
>> The vmalloc region is for *anything which is dynamically mapped*, which
>> includes I/O, vmalloc, and HIGHMEM (kmap).
>>
>>        -hpa
>>
>> --
>> H. Peter Anvin, Intel Open Source Technology Center
>> I work for Intel.  I don't speak on their behalf.
>>
>
>

Thanks Venkatram, do these sound right:

1. All HIGHMEM(physical address beyond 896MB) are kmapped back into
the last 128MB kernel "virtual" address space(using page tables stored
in the last 128MB physical address). That also implies it's a very
limited virtual space for large memory system and need do kunmap when
you're done with it(so you can kmap other physical memories in).
I'm not familiar with large-memory systems, not sure how kmap cope
with that using this limited 128M window assuming kernel is 1:3 split.

2. The last 128MB physical address can be used for page tables(kmap),
vmalloc, IO,etc

Regards,
Xianghua
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