Re: why choose 896MB to the start point of ZONE_HIGHMEM

[H. Peter Anvin]

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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