Re: [PATCH] dma-debug: disable DMA_API_DEBUG for now

[Joerg Roedel]

On Fri, Jun 05, 2009 at 05:33:14PM +0900, FUJITA Tomonori wrote:
> dma-debugs wrongly assumes that no multiple DMA transfers are
> performed against the same dma address on one device at the same
> time. However it's true only with hardware IOMMUs. For example, an
> application can easily send the same buffer twice with different
> lengths to one device by using DIO and AIO. If these requests are not
> unmapped in the same order in which they were mapped,
> hash_bucket_find() finds a wrong entry and gives a false warning.
> 
> We should fix this before 2.6.30 release. Seems that there is no
> easy way to fix it. I think that it's better to just disable
> dma-debug for now.
> 
> Torsten Kaiser found this bug with the RAID1 configuration:
> 
> http://marc.info/?t=124336541900008&r=1&w=2
> 

Argh, I never thought that this can happen. But its not explicitly
forbidden so we have to handle this situation. Thanks for tracking down
the bug to this issue.
However, I think there is a somehow simple fix for the issue. Patch is
attached. Its the least intrusive way I can think of to fix this
problem.
But its up to Linus/Ingo to decide if it can be accepted at this very
late point in the cycle. Since dma-debug is new with 2.6.30 it will at
least not introduce any regression. The patch is boot and basically
load-tested with some disk and network load and showed no issues on my
test machine. The diffstat looks big but more than the half of the patch
adds comments to explain what it does. So the diffstat looks bigger than
the actual change.
Linus, if you think my patch is not acceptable at this point then please
apply the patch from FUJITA Tomonori.

Regards,

	Joerg