Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

From: Tom Talpey
Date: Thu Feb 07 2019 - 10:42:01 EST

On 2/7/2019 10:37 AM, Doug Ledford wrote:
On Thu, 2019-02-07 at 10:28 -0500, Tom Talpey wrote:
On 2/7/2019 10:04 AM, Chuck Lever wrote:

On Feb 7, 2019, at 12:23 AM, Jason Gunthorpe <jgg@xxxxxxxx> wrote:

On Thu, Feb 07, 2019 at 02:52:58PM +1100, Dave Chinner wrote:

Requiring ODP capable hardware and applications that control RDMA
access to use file leases and be able to cancel/recall client side
delegations (like NFS is already able to do!) seems like a pretty

So, what happens on NFS if the revoke takes too long?

NFS distinguishes between "recall" and "revoke". Dave used "recall"
here, it means that the server recalls the client's delegation. If
the client doesn't respond, the server revokes the delegation
unilaterally and other users are allowed to proceed.

The SMB3 protocol has a similar "lease break" mechanism, btw.

SMB3 "push mode" has long-expected to allow DAX mapping of files
only when an exclusive lease is held by the requesting client.
The server may recall the lease if the DAX mapping needs to change.

Once local (MMU) and remote (RDMA) mappings are dropped, the
client may re-request that the server reestablish them. No
connection or process is terminated, and no data is silently lost.

Yeah, but you're referring to a situation where the communication agent
and the filesystem agent are one and the same and they work
cooperatively to resolve the issue. With DAX under Linux, the
filesystem agent and the communication agent are separate, and right
now, to my knowledge, the filesystem agent doesn't tell the
communication agent about a broken lease, it want's to be able to do
things 100% transparently without any work on the communication agent's
part. That works for ODP, but not for anything else. If the filesystem
notified the communication agent of the need to drop the MMU region and
rebuild it, the communication agent could communicate that to the remote
host, and things would work. But there's no POSIX message for "your
file is moving on media, redo your mmap".

Indeed, the MMU notifier and the filesystem need to be integrated.

I'm unmoved by the POSIX argument. This stuff didn't happen in 1990.