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

[Tom Talpey <tom@talpey.com>]

On 2/7/2019 11:57 AM, Ira Weiny wrote:
> 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@ziepe.ca> 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?

The recall process depends on several things, but it certainly takes a
network round trip.

If recall fails, the file protocols allow the server to revoke. However,
since this results in loss of data, it's a last resort.

Tom.