Subject: Re: [2.6 patch] i386: always use 4k stacks
From: Arjan van de Ven <arjan@infradead.org>
To: rol@witbe.net
Cc: "'Kyle Moffett'" <mrmacman_g4@mac.com>, "'Andi Kleen'" <ak@suse.de>,
       "'Adrian Bunk'" <bunk@stusta.de>, akpm@osdl.org,
       linux-kernel@vger.kernel.org
In-Reply-To: <200512172023.jBHKNiD15808@tag.witbe.net>
References: <200512172023.jBHKNiD15808@tag.witbe.net>
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Date: Sat, 17 Dec 2005 21:47:11 +0100
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On Sat, 2005-12-17 at 21:23 +0100, Paul Rolland wrote:
> Hello,
> 
> > One comment on x86-64 vs. x86:  There are restrictions on where in  
> > memory your process stacks can be located on a 32-bit 
> > platform.  They  
> > need to reside in lowmem, which means under certain circumstances  
> > your lowmem can get too fragmented to create new processes even  
> > though you still have a lot of available RAM.
> 
> But where does these restrictions come from ? As far as I know, stack
> is referenced to by SS:ESP registers, and nothing in the x86 architecture
> prevents them from pointing outside of lowmem... Isn't this simply a
> Linux design restriction ?

lowmem is a linux design restriction; only lowmem is directly
addressable.

(also remember that you can have 36 bits of physical ram, but only 32
bit in a pointer, so even if lowmem wasn't 870Mb itd be limited to 4Gb)

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