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

From: Ira Weiny
Date: Thu Feb 07 2019 - 11:58:01 EST


On Thu, Feb 07, 2019 at 10:28:05AM -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.

How long does one wait for these remote mappings to be dropped?

Ira