Messages in this thread | | | Date | Wed, 12 Feb 2014 06:56:36 -0800 | Subject | Re: 3.14-mw regression: rtl8169 WARNING: DMA-API: exceeded 7 overlapping mappings of pfn 55ebe | From | Dan Williams <> |
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On Tue, Feb 11, 2014 at 8:17 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > On Tue, 2014-02-11 at 18:07 -0800, Dan Williams wrote: > >> The overlap granularity is too large. Multiple dma_map_single >> mappings are allowed to a given page as long as they don't collide on >> the same cache line. >> > > I am not sure why you try number of mappings of a page.
For this debug facility I am tracking whether dma has completed by making sure there are no active dma_map entries in the address range of a page being cow'd.
> Try launching 100 concurrent netperf -t TCP_SENFILE > > Same page might be mapped more than 100 times, more than 10000 times in > some cases. >
Aren't these mapping serialized by the device to some extent? Although multi-queue / multi-device would even defeat that...
Hmm, then I think at a minimum the activity tracking needs to be constrained to overlapping DMA_FROM_DEVICE or DMA_BIDIRECTIONAL mappings. However, I am still operating on the assumption that some architectures (especially non-io-coherent or dmabounce architectures) expect a dma mapping to reflect exclusive ownership of the buffer. From the conversation I had with Russell, back in the day [1]:
"When we get to the second async_xor(), as we haven't started to run any of these operations, the source and destination buffers are still mapped. However, we ignore that and call dma_map_page() on them again - this is illegal because the CPU does not own these buffers."
It might be the case that we can't have a general overlap detection facility as it will flag stable use cases that nonetheless violate the exclusivity expectation.
-- Dan
[1]: http://marc.info/?l=linux-arm-kernel&m=129389649101566&w=2
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