Re: the first in a series of memory management questions

[Balaji Rao <balajirrao@xxxxxxxxx>]

On Thursday 03 April 2008 07:55:25 pm Robert P. J. Day wrote:
> 
>   given that i'm determined to nail down how linux MM works, i'm
> perusing the code and some docs from the beginning and, since a lot of
> those docs annoyingly disagree with each other in some fundamental
> places, i'm going to be asking some annoyingly trivial questions.  get
> used to it.  :-)
> 
>   to start, the standard definition of high memory on a 32-bit x86
> system is memory above 896M.
> 
>   * where is that exact boundary defined?
> 
Hi Robert,

Out of the 1G logical address space the kernel sees, 128MB is eaten up by 
VMALLOC_RESERVE for establishing (temporary) mappings into high memory, 
leaving only 896 MB as low memory.

The definition for that 128 MB is found  in arch/x86/mm/init_32.c as 

unsigned int __VMALLOC_RESERVE = 128 << 20;

>   * can it be changed at kernel config time?

Apparently it can't be changed.. 

Does this answer your question ?

regards,
balaji rao

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