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

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Message-ID: <alpine.DEB.2.00.1010150206190.8103@chino.kir.corp.google.com>
Date:	Fri, 15 Oct 2010 02:09:37 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 Thu, 14 Oct 2010, Andrew Morton wrote:

> > We've run with it for a couple years, we can even undefine __GFP_DMA to 
> > find allocations that we compile into the kernel to ensure we don't have a 
> > requirement for the zone.
> 
> "we" is google, I assume.
> 

Yes.

> >  Perhaps only define the gfp flag when we have 
> > CONFIG_ZONE_DMA and break users' builds until they disable options that 
> > require it
> 
> That sounds a good idea.
> 
> What happens in the current patch if someone passes __GFP_DMA when
> CONFIG_ZONE_DMA=n?  ENOMEM?  Perhaps it should WARN() as well.
> 

It'll return NULL and I suggested adding a 

#ifndef CONFIG_ZONE_DMA
	if (WARN_ON_ONCE(gfp_mask & __GFP_DMA))
		return NULL;
#endif

up front in the slowpath.

> hm.  It'd be better to make all the drivers depend on ZONE_DMA.  Good
> luck with that :)
> 

If they all truly depended on it then we'd never have been able to disable 
it.  I think breaking the build when you have CONFIG_ZONE_DMA=n and 
something is passing GFP_DMA would be the best alternative.
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