Why the page tables are needed for lowmem?

Why the page tables are needed for lowmem?

[sandeep kumar]

Hi All

i was going through mm initialization code, and saw paging_init()
implementation.
it has a function map_lowmem().

I was wondering why do we need page table entries for lowmem ??

because all the pages in lowmem can be addressed by kernel logical address
and differ only an offset from the physical address. Why cant we simply add
that offset to the virtual(logical) address and get the job done. Why waste
some memory for creating PTEs for them?

can any one clarify?


-- 
With regards,
Sandeep Kumar Anantapalli,
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Why the page tables are needed for lowmem?

[Pranay Srivastava]

On Thu, Jun 19, 2014 at 8:51 AM, sandeep kumar
<coolsandyforyou@gmail.com> wrote:
> Hi All
>
> i was going through mm initialization code, and saw paging_init()
> implementation.
> it has a function map_lowmem().
>
> I was wondering why do we need page table entries for lowmem ??
>
I hope you understand how MMU and virtual memory works.

> because all the pages in lowmem can be addressed by kernel logical address
> and differ only an offset from the physical address. Why cant we simply add
> that offset to the virtual(logical) address and get the job done. Why waste
> some memory for creating PTEs for them?

Let's forget about lowmem or high mem. See the role of MMU and you'll
understand why it's needed.

>
> can any one clarify?
>
>
> --
> With regards,
> Sandeep Kumar Anantapalli,
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
        ---P.K.S

Why the page tables are needed for lowmem?

[Rik van Riel]

On 06/18/2014 11:21 PM, sandeep kumar wrote:
> Hi All
> 
> i was going through mm initialization code, and saw paging_init()
> implementation.
> it has a function map_lowmem().
> 
> I was wondering why do we need page table entries for lowmem ??
> 
> because all the pages in lowmem can be addressed by kernel logical
> address and differ only an offset from the physical address. Why cant we
> simply add that offset to the virtual(logical) address and get the job
> done. Why waste some memory for creating PTEs for them?
> 
> can any one clarify?

Because after switching the CPU to protected mode,
the CPU can only access memory through page tables,
and can no longer do loads, stores, etc to physical
memory addresses.