Re: [PATCH] [30/30] x86_64: Add missing !X86_PAE dependincy to the 2G/2G split.

[Mark Lord <lkml@rtr.ca>]

William Lee Irwin III wrote:
> On Tue, May 01, 2007 at 05:58:29AM +0200, Andi Kleen wrote:
>> From: ebiederm@xmission.com
>> When in PAE mode we require that the user kernel divide to be
>> on a 1G boundary.  The 2G/2G split does not have that property
>> so require !X86_PAE
>> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
>> ---
>>  arch/i386/Kconfig |    1 +
>>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> What on earth?
> 
> config PAGE_OFFSET
>         hex
>         default 0xB0000000 if VMSPLIT_3G_OPT
>         default 0x78000000 if VMSPLIT_2G
>         default 0x40000000 if VMSPLIT_1G
>         default 0xC0000000
> 
> This appears to have been introduced by:
> commit 975b3d3d5b983eb60706d35f0d24cd19f6badabf
> Author: Mark Lord <lkml@rtr.ca>
> Date:   Wed Feb 1 03:06:11 2006 -0800
>     [PATCH] VMSPLIT config options
> 
> There's some sort of insanity going on here. Since when is 0x78000000
> a 2GB/2GB split? Mark, dare I ask what you were thinking? That should
> be VMSPLIT_2G_OPT for 2GB laptops analogously to VMSPLIT_3G_OPT, if
> nothing else, as it's certainly not 2GB/2GB.

You need to go search the archives and read the *extensive* thread
on this from when it was introduced.  Lots of high profile kernel
developers were in on this one.

The idea is really simple:  eliminate the need for HIGHMEM
on common machines.  

And yes, VMSPLIT_2G really means VMSPLIT_2G_OPT,
in the same way as the (added last) VMSPLIT_3G_OPT flag.

Cheers