linux-kernel - Re: next-0519 on thinkpad x60: sound related? window manager crash

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Date:   Tue, 09 Jun 2020 13:02:46 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     David Rientjes <rientjes@...gle.com>,
        "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>,
        alsa-devel@...a-project.org, bp@...en8.de, hch@...radead.org,
        hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
        Pavel Machek <pavel@....cz>, perex@...ex.cz,
        tglx@...utronix.de, tiwai@...e.com, x86@...nel.org
Subject: Re: next-0519 on thinkpad x60: sound related? window manager crash

On Tue, 09 Jun 2020 11:31:20 +0200,
Takashi Iwai wrote:
> 
> On Tue, 09 Jun 2020 11:17:27 +0200,
> Christoph Hellwig wrote:
> > 
> > On Tue, Jun 09, 2020 at 11:09:14AM +0200, Takashi Iwai wrote:
> > > On Tue, 09 Jun 2020 10:43:05 +0200,
> > > Christoph Hellwig wrote:
> > > > 
> > > > On Tue, Jun 09, 2020 at 10:05:26AM +0200, Takashi Iwai wrote:
> > > > > > >From the disassembly it seems like a vmalloc allocation is NULL, which
> > > > > > seems really weird as this patch shouldn't make a difference for them,
> > > > > > and I also only see a single places that allocates the field, and that
> > > > > > checks for an allocation failure.  But the sound code is a little
> > > > > > hard to unwind sometimes.
> > > > > 
> > > > > It's not clear which sound device being affected, but if it's
> > > > > HD-audio on x86, runtime->dma_area points to a vmapped buffer from
> > > > > SG-pages allocated by dma_alloc_coherent().
> > > > > 
> > > > > OTOH, if it's a USB-audio, runtime->dma_area is a buffer by
> > > > > vmalloc().
> > > > 
> > > > Err, you can't just vmap a buffer returned from dma_alloc_coherent,
> > > > dma_alloc_coherent returns values are opaque and can't be used
> > > > for virt_to_page.  Whatever that code did has already been broken
> > > > per the DMA API contract and on many architectures and just happend
> > > > to work on x86 by accident.
> > > 
> > > Hmm, that's bad.
> > > 
> > > How would be a proper way to get the virtually mapped SG-buffer pages
> > > with coherent memory?  (Also allowing user-space mmap, too)
> > 
> > dma_mmap_coherent / dma_mmap_attrs for userspace.  We don't really
> > have a good way for kernel space mappings.
> 
> And that's the missing piece right now...  :-<

BTW, this kind of usage is not specific to sound, but also V4L also
does vmap() over SG pages from dma_alloc_coherent().  It seems done
only on selected devices, though.


Takashi