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

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Message-ID: <93012d17-d3f9-76d5-e6e0-ea39198db5a9@talpey.com>
Date:   Thu, 7 Feb 2019 16:31:53 -0500
From:   Tom Talpey <tom@...pey.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Chuck Lever <chuck.lever@...cle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Chinner <david@...morbit.com>,
        Doug Ledford <dledford@...hat.com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        lsf-pc@...ts.linux-foundation.org,
        linux-rdma <linux-rdma@...r.kernel.org>, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

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@...pe.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.