On Thu, Oct 12, 2017 at 11:27 AM, Jason Gunthorpe
[off-list ref] wrote:

On Tue, Oct 10, 2017 at 01:17:26PM -0700, Dan Williams wrote:

quoted

Also keep in mind that what triggers the lease break is another
application trying to write or punch holes in a file that is mapped
for RDMA. So, if the hardware can't handle the iommu mapping getting
invalidated asynchronously and the application can't react in the
lease break timeout period then the administrator should arrange for
the file to not be written or truncated while it is mapped.

That makes sense, but why not return ENOSYS or something to the app
trying to alter the file if the RDMA hardware can't support this
instead of having the RDMA app deal with this lease break weirdness?

That's where I started, an inode flag that said "hands off, this file
is busy", but Christoph pointed out that we should reuse the same
mechanisms that pnfs is using. The pnfs protection scheme uses file
leases, and once the kernel decides that a lease needs to be broken /
layout needs to be recalled there is no stopping it, only delaying.

quoted

It's already the case that get_user_pages() does not lock down file
associations, so if your application is contending with these types of
file changes it likely already has a problem keeping transactions in
sync with the file state even without DAX.

Yes, things go weird in non-ODP RDMA cases like this..

Also, just to clear, I would expect an app using the SIGIO interface
to basically halt ongoing RDMA, wait for MRs to become unused locally
and remotely, destroy the MRs, then somehow, establish new MRs that
cover the same logical map (eg what ODP would do transparently) after
the lease breaker has made their changes, then restart their IO.

Does your SIGIO approach have a race-free way to do that last steps?

After the SIGIO that's becomes a userspace / driver problem to quiesce
the I/O...

However, chatting this over with a few more people I have an alternate
solution that effectively behaves the same as how non-ODP hardware
handles this case of hole punch / truncation today. So, today if this
scenario happens on a page-cache backed mapping, the file blocks are
unmapped and the RDMA continues into pinned pages that are no longer
part of the file. We can achieve the same thing with the iommu, just
re-target the I/O into memory that isn't part of the file. That way
hardware does not see I/O errors and the DAX data consistency model is
no worse than the page-cache case.

quoted

quoted

So, not being able to support DAX on certain RDMA hardware is not
an unreasonable situation in our space.

That makes sense, but it still seems to me that this proposed solution
allows more than enough ways to avoid that worst case scenario where
hardware reacts badly to iommu invalidation.

Yes, although I am concerned that returning PCI-E errors is such an
unusual and untested path for some of our RDMA drivers that they may
malfunction badly...

Again, going back to the question of who would ever use this, I would
be very relucant to deploy a production configuration relying on the iommu
invalidate or SIGIO techniques, when ODP HW is available and works
flawlessly.

I don't think it is reasonable to tell people you need to throw away
your old hardware just because you want to target a DAX mapping.

quoted

be blacklisted from supporting DAX altogether. In other words this is
a starting point to incrementally enhance or disable specific drivers,
but with the assurance that the kernel can always do the safe thing
when / if the driver is missing a finer grained solution.

Seems reasonable.. I think existing HW will have an easier time adding
invalidate, while new hardware really should implement ODP.

Yeah, so if we go with 'remap' instead of 'invalidate' does that
address your concerns?

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