On Thu, Aug 24, 2017 at 9:39 AM, Christoph Hellwig [off-list ref] wrote:

On Thu, Aug 24, 2017 at 09:31:17AM -0700, Dan Williams wrote:

quoted

External agent is a DMA device, or a hypervisor like Xen. In the DMA
case perhaps we can use the fcntl lease mechanism, I'll investigate.
In the Xen case it actually would need to use fiemap() to discover the
physical addresses that back the file to setup their M2P tables.
Here's the discussion where we discovered that physical address
dependency:

    https://lists.xen.org/archives/html/xen-devel/2017-04/msg00419.html

fiemap does not work to discover physical addresses.  If they want
to do anything involving physical address they will need a kernel
driver.

True, it's broken with respect to multi-device filesystems and these
patches do nothing to fix that problem. Ok, I'm fine to let that use
case depend on a kernel driver and just focus on fixing the DMA case.