Re: swiotlb_alloc_coherent: allocated memory is out of range for device

[Takashi Iwai]

At Wed, 22 Oct 2008 20:29:24 +0900,
FUJITA Tomonori wrote:
> 
> On Wed, 22 Oct 2008 12:53:58 +0200
> Takashi Iwai <tiwai@xxxxxxx> wrote:
> 
> > At Sun, 19 Oct 2008 12:09:32 +0200,
> > Sven Schnelle wrote:
> > > 
> > > Hi List,
> > > 
> > > my kernel dies while probing parport with the following last words:
> > > 
> > > [    3.672199] parport_pc 00:0b: reported by Plug and Play ACPI
> > > [    3.677969] parport0: PC-style at 0x378 (0x778), irq 7, dma 3 [PCSPP,TRISTATE,COMPAT,EPP,ECP,DMA]
> > > [    3.687691] hwdev DMA mask = 0x0000000000ffffff, dev_addr = 0x0000000020000000
> > > [    3.694916] Kernel panic - not syncing: swiotlb_alloc_coherent: allocated memory is out of range for device
> > > 
> > > I haven't started a bisection yet, but this seems to be introduced
> > > somewhere between 2.6.26 and 2.6.27, at least 2.6.26 was working without
> > > problems. The dmesg log + config was obtained from a kernel compiled
> > > from git on 10/16/2008.
> > 
> > This bug hits me, too.  Looks like swiotlb assumes that the alloc caller
> > must set GFP_DMA appropriately by itself since GFP_DMA hack was
> > removed.  The patch below should fix this particular case.
> 
> This happens with 2.6.27, right? GFP_DMA hack was removed post
> 2.6.27. What kernel version do you hit this problem?

2.6.27 works fine, at least on my machine.
Likely a post-2.6.27 regression.

> Post 2.6.27, x86's alloc_coherent works a bit differently, but neither
> require the caller set to GFP flag. arch/x86/kernel/pci-dma.c does
> with 2.6.27 and asm-x86/dma-mapping.h does with post 2.6.27.
> 
> 
> > HOWEVER: the fundamental problem appears to be in swiotlb itself.
> > It assumes that iotlb pages are in DMA area.  But, in this case, the
> > driver sets 24bit DMA (as of PnP) while iotlb pages are allocated 
> > under 32bit DMA via alloc_bootmem_low_pages().  This doesn't work, of
> > course.
> 
> If a device has 24bit dma mask, alloc_coherent is supposed to use
> GFP_DMA.

Yes.  But what happens if __get_free_pages() fails?  Then you get the
same problem.

> > So, even adding GFP_DMA works mostly, it has still potentially
> > breakage when you can't get the page and fall back to iotlb pages,
> > just like the panic above.
> > 
> > Also, the removal of GFP_DMA hack is a bad idea.  For example, if a
> > device requires 28bit DMA mask, it doesn't set always GFP_DMA for
> > allocation because pages in ZONE_NORMAL may be inside that DMA mask.
> > Normal allocators allow this behavior but swiotlb allocator doesn't.
> > (Correct me if I'm wrong here -- I haven't followed much the recent
> >  changes.)
> 
> 28bit DMA mask is supposed to be handled properly. Firstly, we try
> with DMA_32BIT_MASK and if an allocated address is not fit for 28bit
> mask, we try GFP_DMA again.

Yep, dma_generic_alloc_coherent() works like that for ages.
My point is about swiotlb_alloc_coherent(), and I don't see the
relevant code there...

> > Last but not least, I think panic() in allocation error path is too
> > strict.  Usually returning NULL isn't always fatal error, so give some
> > more chance to debug, e.g. by calling WARN() (or whatever) instead of
> > panic().
> 
> Yeah, this was discussed several times. The problem is that many
> drivers assume that dma mapping operations, map_single, map_sg, and
> map_coherent, always succeed and doesn't even check the errors. So we
> have some panic() in IOMMU drivers to prevent really bad events like
> data corruption.

Well, but also for alloc_coherent()?  Returning NULL from
dma_alloc_coherent() is really no fatal error.  If the caller doesn't
check the return value, then it's a more serious bug, I'd say.


thanks,

Takashi
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