linux-kernel - Re: [PATCH] Make GFP_DMA allocations w/o ZONE

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Message-ID: <20110610220748.GO24424@n2100.arm.linux.org.uk>
Date:	Fri, 10 Jun 2011 23:07:48 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	tglx@...utronix.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mel@....ul.ie,
	kamezawa.hiroyu@...fujitsu.com, riel@...hat.com, pavel@....cz
Subject: Re: [PATCH] Make GFP_DMA allocations w/o ZONE_DMA emit a warning
	instead of failing

On Fri, Jun 10, 2011 at 03:01:15PM -0700, David Rientjes wrote:
> On Fri, 10 Jun 2011, Russell King - ARM Linux wrote:
> > See Linus' reply.  I quote again "on platforms where it doesn't matter it
> > should be a no-op".  If _you_ have a problem with that _you_ need to
> > discuss it with _Linus_, not me.  I'm not going to be a middle-man sitting
> > between two people with different opinions.
> 
> We're talking about two different things.  Linus is saying that if GFP_DMA 
> should be a no-op if the hardware doesn't require DMA memory because the 
> kernel was correctly compiled without CONFIG_ZONE_DMA.  I'm asking about a 
> kernel that was incorrectly compiled without CONFIG_ZONE_DMA and now we're 
> returning memory from anywhere even though we actually require GFP_DMA.

How do you distinguish between the two states?  Answer: you can't.

> If you don't want to form an opinion of your own, then I have no problem 
> cc'ing Linus on it.

I think I've made my position in this fairly clear with our previous
discussions on this.  The fact of the matter is that there are some
drivers which use GFP_DMA because they need that on _some_ platforms
but not all.

Not all platforms are _broken_ with respect to DMA, and those which
aren't broken don't provide a DMA zone.

Therefore, it is _perfectly_ _legal_ to honor a GFP_DMA allocation on
a kernel without CONFIG_ZONE_DMA set.

That position appears to be reflected by Linus' response, so

>  I don't think he'd object to a
> 
> 	#ifndef CONFIG_ZONE_DMA
> 	WARN_ON_ONCE(1, "%s (%d): allocating DMA memory without DMA support -- "
> 			"enable CONFIG_ZONE_DMA if needed.\n",
> 			current->comm, current->pid);
> 	#endif

I think he will definitely object to this on the grounds that its
adding useless noise for conditions which are _perfectly_ _valid_.
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