Re: [PATCH v9 0/6] MAP_DIRECT for DAX userspace flush

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On Mon, Oct 16, 2017 at 10:43 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Mon, Oct 16, 2017 at 12:26 AM, Christoph Hellwig <hch@xxxxxx> wrote:
>> On Fri, Oct 13, 2017 at 11:31:45AM -0600, Jason Gunthorpe wrote:
>>> I don't think that really represents how lots of apps actually use
>>> RDMA.
>>>
>>> RDMA is often buried down in the software stack (eg in a MPI), and by
>>> the time a mapping gets used for RDMA transfer the link between the
>>> FD, mmap and the MR is totally opaque.
>>>
>>> Having a MR specific notification means the low level RDMA libraries
>>> have a chance to deal with everything for the app.
>>>
>>> Eg consider a HPC app using MPI that uses some DAX aware library to
>>> get DAX backed mmap's. It then passes memory in those mmaps to the
>>> MPI library to do transfers. The MPI creates the MR on demand.
>>>
>>
>> I suspect one of the more interesting use cases might be a file server,
>> for which that's not the case.  But otherwise I agree with the above,
>> and also thing that notifying the MR handle is the only way to go for
>> another very important reason:  fencing.  What if the application/library
>> does not react on the notification?  With a per-MR notification we
>> can unregister the MR in kernel space and have a rock solid fencing
>> mechanism.  And that is the most important bit here.
>
> While I agree with the need for a per-MR notification mechanism, one
> thing we lose by walking away from MAP_DIRECT is a way for a
> hypervisor to coordinate pass through of a DAX mapping to an RDMA
> device in a guest. That will remain a case where we will still need to
> use device-dax. I'm fine if that's the answer, but just want to be
> clear about all the places we need to protect a DAX mapping against
> RDMA from a non-ODP device.

For this specific issue perhaps we promote FL_LAYOUT as a lease-type
that can be set by fcntl().
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