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

[ebiederm@xmission.com (Eric W. Biederman)]

William Lee Irwin III <wli@holomorphy.com> writes:

> 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.

It makes a little more sense when you realize all of the options
were originally !X86_PAE.  So they were designed with highmem
disabled.

> These VMSPLIT config options vs. PAE are foul as they're now done in
> any event. If they were done properly, they'd properly set up the pmd
> within which the division point between user and kernelspace falls.

They were designed to avoid highmem a totally different design point.

> This patch, I suppose, stops people from shooting themselves in the
> foot, but (IMHO) the VMSPLIT patches shouldn't have been merged
> without handling the partial pmd case. 2MB/4MB resolution is enough
> granularity for any reasonable purpose, so split ptes aren't worth the
> effort, but this nonsense with PAE vs. VMSPLIT is just preposterous.
> If you're going to play the VMSPLIT game at all, handle split pmd's.

What I find telling is that I fixed this based on code review not
based on bug reports.

> I'll see what else is pending in the i386 pagetable arena and clear
> this up if there aren't other objections (this is where Andi gets to
> complain that things are too complex already and preemptively NAK to
> save me the effort, if it's not seen to be desirable). Eric, your patch
> is a reasonable stop-gap measure for the original deficiency.

Frankly rather then putting much effort into this I suspect we should
just delete these options entirely.  We are long past the point where
they matter.

Eric