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

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Message-ID: <20110610222020.GP24424@n2100.arm.linux.org.uk>
Date:	Fri, 10 Jun 2011 23:20:20 +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 03:16:00PM -0700, David Rientjes wrote:
> On Fri, 10 Jun 2011, Russell King - ARM Linux wrote:
> 
> > > 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.
> > 
> 
> By my warning which says "enable CONFIG_ZONE_DMA _if_ needed."  The 
> alternative is to silently return memory from anywhere, which is what the 
> page allocator does now, which doesn't seem very user friendly when the 
> device randomly works depending on the chance it was actually allocated 
> from the DMA mask.  If it actually wants DMA and the kernel is compiled 
> incorrectly, then I think a single line in the kernel log would be nice to 
> point them in the right direction.  Users who disable the option usually 
> know what they're doing (it's only allowed for CONFIG_EXPERT on x86, for 
> example), so I don't think they'll mind the notification and choose to 
> ignore it.

So those platforms which don't have a DMA zone, don't have any problems
with DMA, yet want to use the very same driver which does have a problem
on ISA hardware have to also put up with a useless notification that
their kernel might be broken?

Are you offering to participate on other architectures mailing lists to
answer all the resulting queries?
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