linux-kernel - Re: [PATCH] [30/30] x86_64: Add missing !X86

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Date:	Mon, 30 Apr 2007 21:37:35 -0700
From:	William Lee Irwin III <wli@...omorphy.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Mark Lord <lkml@....ca>, ebiederm@...ssion.com, patches@...-64.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [30/30] x86_64: Add missing !X86_PAE dependincy to the 2G/2G split.

On Tue, May 01, 2007 at 05:58:29AM +0200, Andi Kleen wrote:
> From: ebiederm@...ssion.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@...ssion.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@....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.

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.

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.

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.

-- wli
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