On Wed, Apr 27, 2011 at 02:42:57PM +0300, saeed bishara wrote: > On Sun, Apr 17, 2011 at 7:00 PM, saeed bishara <saeed.bishara@xxxxxxxxx> wrote: > > Hi, > > when md uses the dma for offloading xor and memcpy operations, it > > violates the dma-mapping API. here is the scenario I'm taking about > > (under write to degraded raid5): > > 1. ops_run_prexor sends xor operation from buffers A and B, and the > > destination is A. > > 2. ops_run_biodrain: sends mempcy operation from C to B. > > 3. ops_run_reconstruct5: sends xor operation from A and B, and the > > destination is A again. > > > > in step 1, the async tx maps A using dma_map_page, and in step 3, it > > maps again the same buffer. but, if the request from step 1 still > > being handled the dma engine, then we end with a case where the buffer > > mapped while it still belongs to the dma hw. > > when the arch is ARMv6/SMP mode (without io coherency), the cache > > maintenance involves read/write access to the buffers, that means, the > > second mapping above may access the buffer(with read/write) while the > > dma is writing to it!!. > > > Russell/Dan, > can you have a look into this issue? what I see here is that the > raid stack issues dma_map_page to a buffer that still owned by DMA. I already mentioned this issue to Dan, and pointed out that it's a violation of the buffer ownership rules. I don't remember clearly what the outcome of it is, but there's not a lot which can be done at architecture level about it. I think the buffer mapping was going to be moved upwards, to prevent the multiple buffer mapping issue. I don't know if patches were produced though (and I don't have hardware to be able to produce and test such patches against.) -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
