RE: Support for HIGHMEM ...can any one explain my Q&A ..

RE: Support for HIGHMEM ...can any one explain my Q&A ..

[Mukund JB.]

Thanks a lot for the response.
Will u be able to provide me a link that will be able to help in making
this PAGE-OFFSET and related issues more clear.

Also, it seems to be whats going on here is high-end expertise level
stuff.

I am new to Driver Writing even though I was working on linux before
this. So, will u be able to suggest me a list where in I can post my
basic issues about Memory management issues, Time Control issues and
Driver Control, so on.

Or else is it ok if I post them here.


Thanks for that again.

Regards,
Mukund jampala


-----Original Message-----
From: eric@ebiederm.dsl.xmission.com
[mailto:eric@ebiederm.dsl.xmission.com] On Behalf Of Eric W. Biederman
Sent: Monday, August 23, 2004 7:59 PM
To: Mukund JB.
Cc: linux-kernel@vger.kernel.org
Subject: Re: Support for HIGHMEM ...can any one explain my Q&A ..

"Mukund JB." <mukundjb@esntechnologies.co.in> writes:

> Hi all,
> Sorry if I asked any thing that basic level stuff?
> This is the paasge I found in the net while surfing for details about
> HIGHMEM stuff.
> 
> During 2.3 kernel development (I think), "HIGHMEM" support was added.
> Normally, the kernel can only address (4GB-PAGE_OFFSET)/PAGE_SIZE
pages
> of RAM, since all physical pages must be mapped to kernel addresses
> between PAGE_OFFSET and 4GB. (So if PAGE_OFFSET is 3GB, only 1GB of
> physical RAM can be used - not even that, in practice, due to fixed
> kernel mappings and so forth.) The HIGHMEM patches allow the kernel to
> use more than 1G of memory by mapping the additional pages into the
high
> part of the kernel address space just below 4GB as necessary. They
also
> allow high-memory pages to be mapped into user process address space.
> 
> 
> ***) Does the above passage mean that PAGE_OFFSET is the starting
> address of my RAM ?

No.  PAGE_OFFSET is the virtual address at which physical address
0 is mapped into an arch/i386 kernel.  

> I understood so from the below line
> "since all physical pages must be mapped to kernel addresses between
> PAGE_OFFSET and 4GB".
> 
> ***) Does it mean that the lowest portion of the 4GB os nothing but
RAM
> ?

No.  0-PAGE_OFFSET is the virtual address space provided to user space.
So it has an arbitrary mapping to virtual memory.



Eric

RE: Support for HIGHMEM ...can any one explain my Q&A ..

[Dave Hansen]

On Mon, 2004-08-23 at 21:10, Mukund JB. wrote:
> Thanks a lot for the response.
> Will u be able to provide me a link that will be able to help in making
> this PAGE-OFFSET and related issues more clear.

Just search the list archives or google for it.  I don't think there are
any web pages on that subject alone.  

> I am new to Driver Writing even though I was working on linux before
> this. So, will u be able to suggest me a list where in I can post my
> basic issues about Memory management issues, Time Control issues and
> Driver Control, so on.

Go ahead and ask them here.  And please quote message _above_ your
reply, not below.

-- Dave

Support for HIGHMEM ...can any one explain my Q&A ..

[Mukund JB.]

Hi all,
Sorry if I asked any thing that basic level stuff?
This is the paasge I found in the net while surfing for details about
HIGHMEM stuff.

During 2.3 kernel development (I think), "HIGHMEM" support was added.
Normally, the kernel can only address (4GB-PAGE_OFFSET)/PAGE_SIZE pages
of RAM, since all physical pages must be mapped to kernel addresses
between PAGE_OFFSET and 4GB. (So if PAGE_OFFSET is 3GB, only 1GB of
physical RAM can be used - not even that, in practice, due to fixed
kernel mappings and so forth.) The HIGHMEM patches allow the kernel to
use more than 1G of memory by mapping the additional pages into the high
part of the kernel address space just below 4GB as necessary. They also
allow high-memory pages to be mapped into user process address space.


***) Does the above passage mean that PAGE_OFFSET is the starting
address of my RAM ?
I understood so from the below line
"since all physical pages must be mapped to kernel addresses between
PAGE_OFFSET and 4GB".

***) Does it mean that the lowest portion of the 4GB os nothing but RAM
?

thanks in advance Guys,

regards,
Mukund Jampala

Re: Support for HIGHMEM ...can any one explain my Q&A ..

[Eric W. Biederman]

"Mukund JB." <mukundjb@esntechnologies.co.in> writes:

> Hi all,
> Sorry if I asked any thing that basic level stuff?
> This is the paasge I found in the net while surfing for details about
> HIGHMEM stuff.
> 
> During 2.3 kernel development (I think), "HIGHMEM" support was added.
> Normally, the kernel can only address (4GB-PAGE_OFFSET)/PAGE_SIZE pages
> of RAM, since all physical pages must be mapped to kernel addresses
> between PAGE_OFFSET and 4GB. (So if PAGE_OFFSET is 3GB, only 1GB of
> physical RAM can be used - not even that, in practice, due to fixed
> kernel mappings and so forth.) The HIGHMEM patches allow the kernel to
> use more than 1G of memory by mapping the additional pages into the high
> part of the kernel address space just below 4GB as necessary. They also
> allow high-memory pages to be mapped into user process address space.
> 
> 
> ***) Does the above passage mean that PAGE_OFFSET is the starting
> address of my RAM ?

No.  PAGE_OFFSET is the virtual address at which physical address
0 is mapped into an arch/i386 kernel.  

> I understood so from the below line
> "since all physical pages must be mapped to kernel addresses between
> PAGE_OFFSET and 4GB".
> 
> ***) Does it mean that the lowest portion of the 4GB os nothing but RAM
> ?

No.  0-PAGE_OFFSET is the virtual address space provided to user space.
So it has an arbitrary mapping to virtual memory.

Eric