Message-Id: <20081022221321J.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 22 Oct 2008 22:13:39 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	tiwai@...e.de
Cc:	fujita.tomonori@....ntt.co.jp, svens@...ckframe.org,
	joerg.roedel@....com, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: swiotlb_alloc_coherent: allocated memory is out of range for
 device

On Wed, 22 Oct 2008 14:06:48 +0200
Takashi Iwai <tiwai@...e.de> wrote:

> At Wed, 22 Oct 2008 20:29:24 +0900,
> FUJITA Tomonori wrote:
> > 
> > On Wed, 22 Oct 2008 12:53:58 +0200
> > Takashi Iwai <tiwai@...e.de> 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.

Ok, it makes sense because I don't see any major changes to swiotlb
between 2.6.26 and 2.6.27.


> > 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.

Yeah, but __get_free_pages() with GFP_DMA fails, what can we do in
such case? You think that it's a good idea to change swiotlb to
allocates < 16MB iotlb pages? I'm not sure it's worth to do
that. 24bit dma mask devices are disappearing.

About the bug that you hit, I suspect that dma_map_coherent() in
asm-x86/dma-mapping.h doesn't set gfp flags correctly.

dma_map_coherent() calls swiotlb_alloc_coherent with the flags GFP_DMA
set? parport driver set dev->coherent_dma_mask properly?


> > > 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...

Oops, you are right. swiotlb doesn't try again with GFP_DMA now. Joerg
changed the GFP_DMA retry mechanism work only for pci-nommu.c It broke
GART IOMMU and x86's swiotlb. I modified dma_generic_alloc_coherent to
work with pci-nommu and GART. I promised Ingo to fix swiotlb too but I
forgot about it.

Sorry, I'll fix this soon but your case (28bit mask) is supposed to
work without the GFP_DMA retry mechanism. As I wrote above, I suspect
that dma flag is not set correctly.


> > > 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.

Probably, the majority check dma_alloc_coherent failure. I'll check
this later to remove panic() in alloc_coherent in IOMMUs.
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