mem_map

mem_map

[Vaibhav Sharma]

hi all,
Can any body provide any information for the significance of
"CONFIG_VIRTUAL_MEM_MAP"?

TIA, 
Vaibhav

Re: mem_map

[Grant Grundler]

On Fri, May 06, 2005 at 07:59:59PM +0530, Vaibhav Sharma wrote:
> hi all,
> Can any body provide any information for the significance of
> "CONFIG_VIRTUAL_MEM_MAP"?

	help
	  Say Y to compile the kernel with support for a virtual mem map.
	  This code also only takes effect if a memory hole of greater than
	  1 Gb is found during boot.  You must turn this option on if you
	  require the DISCONTIGMEM option for your machine. If you are
	  unsure, say Y.

The first sentence could define what a virtual mem map is.
IIRC, it's something to do with non-contiguous page table.

grant

Re: mem_map

[David Mosberger]

>>>>> On Fri, 6 May 2005 19:59:59 +0530, Vaibhav Sharma <vaibhav.sh@gmail.com> said:

  Vaibhav> hi all, Can any body provide any information for the
  Vaibhav> significance of "CONFIG_VIRTUAL_MEM_MAP"?

Turning on this option puts the Linux physical memory-map (mem_map)
into virtual memory rather than physical memory.  Doing so can be an
advantage on machines which have holes in the physical memory.  For
example, on zx1-based machines, physical memory 1GB-4GB shows up at an
address above 257GB so there is a hole of about 256GB below that
address.

There are other ways to manage (huge) holes in physical memory space,
but with the large page-sizes (64MB or 16MB) we use on ia64 for
identity-mapping kernel space, this approach seems to work quite well.

	--david