Re: 2G memory split

[Martin Bligh <mbligh@mbligh.org>]

Linus Torvalds wrote:
> 
> On Tue, 10 Jan 2006, Jens Axboe wrote:
> 
>>A newer version, trying to cater to the various comments in here.
>>Changes:
> 
> 
> Can we do one final cleanup? Do all the magic in _one_ place, namely the 
> x86 Kconfig file.
> 
> Also, I don't think the NOHIGHMEM dependency is necessarily correct. A 
> 2G/2G split can be advantageous with a 16GB setup (you'll have more room 
> for dentries etc), but you obviously want to have HIGHMEM for that..
> 
> Do it something like this:
> 
> 	choice
> 		depends on EXPERIMENTAL
> 		prompt "Memory split"
> 		default DEFAULT_3G
> 		help
> 		  Select the wanted split between kernel and user memory.
> 		  If the address range available to the kernel is less than the
> 		  physical memory installed, the remaining memory will be available
> 		  as "high memory". Accessing high memory is a little more costly
> 		  than low memory, as it needs to be mapped into the kernel first.
> 		  Note that selecting anything but the default 3G/1G split will make
> 		  your kernel incompatible with binary only modules.
> 
> 		config DEFAULT_3G
> 			bool "3G/1G user/kernel split"
> 		config DEFAULT_3G_OPT
> 			bool "3G/1G user/kernel split (for full 1G low memory)"
> 		config DEFAULT_2G
> 			bool "2G/2G user/kernel split"
> 		config DEFAULT_1G
> 			bool "1G/3G user/kernel split"
> 	endchoice
> 
> 	config PAGE_OFFSET
> 		hex
> 		default 0xC0000000
> 		default 0xB0000000 if DEFAULT_3G_OPT
> 		default 0x78000000 if DEFAULT_2G
> 		default 0x40000000 if DEFAULT_1G
> 
> and then asm-i386/page.h can just do
> 
> 	#define __PAGE_OFFSET ((unsigned long)CONFIG_PAGE_OFFSET)
> 
> and you're done.

The non-1GB-aligned ones need to be disbarred when PAE is on, I think.

M.