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

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Message-ID: <alpine.DEB.2.00.1010131744540.27124@chino.kir.corp.google.com>
Date:	Wed, 13 Oct 2010 17:49:53 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Ingo Molnar <mingo@...hat.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, H. Peter Anvin wrote:

> > You can already set the lowmem_reserve_ratio via 
> > /proc/sys/vm/lowmem_reserve_ratio.
> 
> And why is it then so hard to put in a single line in /etc/sysctl.conf
> for you?
> 

Because disabling CONFIG_ZONE_DMA allows for additional extensions: 
disabling CONFIG_GENERIC_ISA_DMA and CONFIG_ISA_DMA_API.  We've also run 
for a couple years without those.

> > We've run without ZONE_DMA for a couple years because we don't have such 
> > hardware limitations and would appreciate the ability to disable it with a 
> > config option rather than hacking the kernel to get it to compile.  I'm 
> > sure Casey would as well as you can see in the thread "A question about 
> > ZONE_DMA".
> > 
> > I don't see the harm in being able to conveniently disable the zone if you 
> > know what you're doing.
> 
> There are going to be a lot of very strange bug reports as a result.
> 

We have config options all over the kernel that change the behavior of 
syscalls without worrying about bug reports popping up from users who 
knowingly have disabled the option, I don't see this as being any 
different.  If you're concerned about, we could add a

#ifndef CONFIG_ZONE_DMA
	WARN_ON_ONCE(gfp_mask & __GFP_DMA);
#endif

in the page allocator.
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