On Fri, Oct 6, 2017 at 3:45 PM, David Woodhouse [off-list ref] wrote:

On Fri, 2017-10-06 at 15:35 -0700, Dan Williams wrote:

quoted

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.