On Fri, 2017-10-06 at 15:52 -0700, Dan Williams wrote: > On Fri, Oct 6, 2017 at 3:45 PM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote: > > > > On Fri, 2017-10-06 at 15:35 -0700, Dan Williams wrote: > > > > > > Add a helper to determine if the dma mappings set up for a given device > > > are backed by an iommu. In particular, this lets code paths know that a > > > dma_unmap operation will revoke access to memory if the device can not > > > otherwise be quiesced. The need for this knowledge is driven by a need > > > to make RDMA transfers to DAX mappings safe. If the DAX file's block map > > > changes we need to be to reliably stop accesses to blocks that have been > > > freed or re-assigned to a new file. > > "a dma_unmap operation revoke access to memory"... but it's OK that the > > next *map* will give the same DMA address to someone else, right? > > I'm assuming the next map will be to other physical addresses and a > different requester device since the memory is still registered > exclusively. I meant the next map for this device/group. It may well use the same virtual DMA address as the one you just unmapped, yet actually map to a different physical address. So if the DMA still occurs to the "old" address, that isn't revoked at all — it's just going to the wrong physical location. And if you are sure that the DMA will never happen, why do you need to revoke the mapping in the first place?
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