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?