On Tue, Feb 11, 2014 at 8:17 PM, Eric Dumazet <eric.duma...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
