From: William Lee Irwin III <[email protected]>
>> 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.
On Thu, Jul 19, 2007 at 03:52:29PM +0100, Christoph Hellwig wrote:
> 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.
The missing context here is CONFIG_VMSPLIT on laptops.
Laptop users, who frequently use CONFIG_VMSPLIT options to avoid
highmem, wanted to turn on NX. Prior to the patch, those options were
barred for all highmem configurations. In response to those requests,
I produced the patch.
The overhead and instability derived from tiny zones as opposed to
kmap()/kunmap(), or at least such was the case historically.
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