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

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Message-ID: <4BBCBD8B.6070701@zytor.com>
Date:	Wed, 07 Apr 2010 10:14:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Himanshu Aggarwal <lkml.himanshu@...il.com>
CC:	Chetan Nanda <chetannanda@...il.com>,
	Xianghua Xiao <xiaoxianghua@...il.com>,
	Venkatram Tummala <venkatram867@...il.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 04/07/2010 09:48 AM, Himanshu Aggarwal wrote:
> I think for some architectures, the position of highmem is constrained
> by hardware as well.  It is not always a kernel decision and not always
> configurable as in case of x86.


This is correct.

> In case of MIPS32, low memory is between 0 and 512 MB and high memory
> starts above 512 MB. Also the user space is of size 2 GB. 
> 
> Please see the definition of macros PAGE_OFFSET and HIGHMEM_START at :
> http://lxr.linux.no/linux+v2.6.33/arch/mips/include/asm/mach-generic/spaces.h

Right so far...

> This is because MIPS32 processors have KSEG0 and KSEG1 segments lying
> between 0 and 512 MB and KSEG2/3 lies above it. 
> 
> May be someone on the group can confirm this.

Wrong.  I have to say this thread has been just astonishing in the
amount of misinformation.

On MIPS32, userspace is 0-2 GB, kseg0 is 2.0-2.5 GB and kseg1 is 2.5-3.0
GB.  kseg2/3 (3.0-4.0 GB), which invokes the TLB, is used for the
vmalloc/iomap/kmap area.

LOWMEM has to fit inside kseg0, so LOWMEM is limited to 512 MB in thie
current Linux implementation.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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