Mapping of ZONE_HIGHMEM in kernel address space in 32bit x86

[Prabhu nath]

Is this is a question that popped up to your mind arbitrarily or do you
have a specific system at hand which triggered you to ponder over the
design of the kernel ? I felt the answer to this question is not straight
forward but is multi faceted and to be discussed in a specific context.


On Sat, May 11, 2013 at 9:02 AM, Paul Davies C <pauldaviesc at gmail.com>wrote:

>
> In a system with 3:1 split, the ZONE_NORMAL with a size of 896MB is
> permanently mapped to the kernel address space.This leaves a 128MB free
> space in the Kernel address space and according to my understanding, the
> ZONE_HIGHMEM pages are mapped temporarily to this 128MB part. If the system
> actually had a 4GB physical memory you will be mapping(not smultaneously)
> the HIHGMEM part- which is roughly 3.2GB - to this 128MB part. If that was
> the case Kernel would have to frequently access HIHGMEM which implicates a
> frequent change in temporaty mapping and that in my view is a penalty. So
> what was the reason why ZONE_NORMAL fixed at 896MB and not something really
> lower?
>
> --
> *Regards,*
> *Paul Davies C*
> vivafoss.blogspot.com
>
>
-- 
Regards,
Prabhunath G
Linux Trainer
Bangalore
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