linux-kernel - Re: [PATCH for review] [7/48] i386: divorce CONFIG_X86_PAE from CONFIG

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Message-ID: <20070719145229.GA19858@infradead.org>
Date:	Thu, 19 Jul 2007 15:52:29 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Andi Kleen <ak@...e.de>
Cc:	wli@...omorphy.com, lkml@....ca, linux-kernel@...r.kernel.org
Subject: Re: [PATCH for review] [7/48] i386: divorce CONFIG_X86_PAE from CONFIG_HIGHMEM64G

On Thu, Jul 19, 2007 at 03:48:36PM +0200, Andi Kleen wrote:
> 
> From: William Lee Irwin III <wli@...omorphy.com>
> 
> PAE is useful for more than supporting more than 4GB RAM.  It supports
> expanded swapspace and NX executable protections.  Some users may want NX
> or expanded swapspace support without the overhead or instability of
> highmem.  For these reasons, the following patch divorces CONFIG_X86_PAE
> from CONFIG_HIGHMEM64G.

What overhead of instability of highmem?  Sorry folks but this is utter
bollocks.  Back in the Caldera days we did a lot of measurement on highmem
overhead, and CONFIG_HIGHMEM has no measurable overhead at all on a system
that doesn't use it.  CONFIG_HIGHMEM64G on the other hand has
a quite visible overhead on small systems, but that's entirely due to the
bigger page table entries that you need for NX.

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