iommu/arm-smmu: Regression (sleeping function called from invalid context)

[Andreas Herrmann]

On Fri, Jan 31, 2014 at 09:46:23AM +0100, Andreas Herrmann wrote:
> On Fri, Jan 31, 2014 at 12:55:52AM +0100, Andreas Herrmann wrote:
> > Hi Will,
> > 
> > Seems that commit a44a9791e778d9ccda50d5534028ed4057a9a45b
> > (iommu/arm-smmu: use mutex instead of spinlock for locking page tables)
> > introduced a regression.
> > 
> > At least I've hit
> > 
> >   BUG: scheduling while atomic: ksoftirqd/0/3/0x00000100
> >...
> 
> >   BUG: sleeping function called from invalid context at mm/page_alloc.c:2679
> >   in_atomic(): 1, irqs_disabled(): 128, pid: 0, name: swapper/0
> >   CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.13.0-00016-g6e90346 #413
> >   [<c0014740>] (unwind_backtrace+0x0/0xf8) from [<c00115b0>] (show_stack+0x10/0x14)
> >   [<c00115b0>] (show_stack+0x10/0x14) from [<c057ea24>] (dump_stack+0x74/0xa8)
> >   [<c057ea24>] (dump_stack+0x74/0xa8) from [<c00acc1c>] (__alloc_pages_nodemask+0x174/0x930)
> >   [<c00acc1c>] (__alloc_pages_nodemask+0x174/0x930) from [<c042a250>] (arm_smmu_handle_mapping+0x470/0x66c)
> >   [<c042a250>] (arm_smmu_handle_mapping+0x470/0x66c) from [<c0428e74>] (iommu_map+0xf0/0x148)
> >   [<c0428e74>] (iommu_map+0xf0/0x148) from [<c001935c>] (__map_sg_chunk+0x198/0x2d4)
> >...
> 
> > Maybe that was the reason why the offending commit was introduced(?).
> > 
> > I think with the current code "atomic allocations" should be used when
> > IO page tables are created. With below patch I've not triggered above
> > errors.
> 
> I think allocating memory with GFP_KERNEL in this dma-mapping path
> doesn't seem to be a good idea. What if the DMA operation for which we
> modify IO page tables was triggered to free pages (page cache, swap)?

I mean in case we run out of memory wouldn't we worsen the situation
by triggering additional IO (and thus DMA)? Whereas when we let the
mapping fail, the OS "just might have to wait a little bit" until
other DMA activities are completed, pages unmapped and iova freed. The
freed resources instantly can be used for further DMA activities.

Hmm, but maybe I need to rethink this (and look more closely into
page_alloc.c).


Andreas