linux-kernel - Re: [patch] x86: allow ZONE

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Date:	Thu, 14 Oct 2010 01:10:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Casey Dahlin <cdahlin@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] x86: allow ZONE_DMA to be configurable

On Wed, 13 Oct 2010 17:15:04 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> ZONE_DMA is unnecessary for a large number of machines that do not
> require addressing in the lower 16MB of memory because they do not use
> ISA devices with 16-bit address registers (plus one page byte register).
> 
> This patch allows users to disable ZONE_DMA for x86 if they know they
> will not be using such devices with their kernel.
> 
> This prevents the VM from unnecessarily reserving a ratio of memory
> (defaulting to 1/256th of system capacity) with lowmem_reserve_ratio
> for such allocations when it will never be used.
> 

I wonder how hard it would be to do this at runtime, probably with a
boot parameter.

I'd be a little concerned at the effects of this on page reclaim and
the page allocator - it might expose weird pre-existing bugs or
inefficiencies.  But we can cross that bridge when we fall off it, I
guess.

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