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

From: H. Peter Anvin
Date: Wed Apr 07 2010 - 01:38:25 EST

On 04/06/2010 09:05 PM, Chetan Nanda wrote:
> I have a question here, what if I have a 32bit system with 2GB of RAM,
> so in that case my 896MB - 2GB RAM would be in accessible?
> What my understanding on the subject is:
> Only 896 MB of physical RAM is directly mapped on to kernel 1G virtual
> address space. And we still require page table settings to do that (page
> table would be identity mapping). But for rest of RAM, i.e. whenever
> there is need to access physical RAM beyond 896MB then that page will be
> mapped on to pages from 128MB kernel virtual address space (1GB - 896MB
> = 128MB), and AFAIK kmap is just for that.
> Please correct me if i am wrong


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

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