linux-kernel - Re: [BUG] pgprot_noncached() is -NOT- safe for mapping vmalloc buffers into userspace

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Date:	Fri, 25 Mar 2011 21:23:26 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	linux-arch@...r.kernel.org, Matthew Evans <matt.evans@....ibm.com>
Subject: Re: [BUG] pgprot_noncached() is -NOT- safe for mapping vmalloc
 buffers into userspace

On Fri, 2011-03-25 at 11:12 +0100, Takashi Iwai wrote:
> At Fri, 25 Mar 2011 20:15:33 +1100,
> Benjamin Herrenschmidt wrote:
> > 
> > > > We
> > > > must also make sure we don't go down that path for vmalloc memory
> > > > though.
> > >
> > > Yes.
> > 
> > I haven't actually checked, but I assume that the test
> > 
> > substream->dma_buffer.dev.type == SNDRV_DMA_TYPE_DEV
> > 
> > In snd_pcm_default_mmap() takes care of that, please correct me if
> > I'm wrong in which case we'll need something else there.
> 
> Well, in the case of usb-audio, it's not handled via
> dma_mmap_coherent(), as the page isn't allocated via
> dma_alloc_coherent() but vmalloc().

Right, I just wanted to make sure I was right to assume that a page
allocated by vmalloc() was going to fail the above test in 
snd_pcm_default_mmap() and thus -not- get into dma_mmap_coherent()...
just double checking as I'm not totally familiar with the intricacies of
the pcm code :-)

> The bad commit was to overcome some problems on SH platform, IIRC,
> when it's used with dmix -- i.e. concurrent accesses on the mmapped
> buffer from multiple processes.  But, this looks obviously like a
> wrong approach.

Is this a vivt architecture ? Maybe enforcing some restrictions on the
virtual addresses so they hit the same cache congruence classes ?

> Actually, the buffer allocated there in usb-audio is an intermediate
> buffer, that isn't directly transferred to hardware.  We may need a
> bit more consideration what is the best way to solve that issue (if
> it's still really present).

Right. I wouldn't expect vmalloc stuff to hit HW in most cases anyways,
though I do wonder why you don't pass the buffer directly to the HCD and
avoid that intermediate step but that's a completely different
question :-)

> > > Your patch looks good.  Thanks for taking care of this! 
> > 
> > Are you taking care of sending it upstream ?
> 
> I'll apply the patch to remove vmalloc pgprot thingy surely to sound
> tree and include in the next pull request.
> 
> Others should be sent through arch maintainers (PPC and ARM), right?

Well, I am ppc maintainer so that's sorted :-) I've CCed Russell for the
other, it's up to him, I have no specific dependency there, it's just an
easy cleanup I stumbled upon.

Cheers,
Ben.

> 
> thanks,
> 
> Takashi
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