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

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Message-ID: <20190206173114.GB12227@ziepe.ca>
Date:   Wed, 6 Feb 2019 10:31:14 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jan Kara <jack@...e.cz>
Cc:     Ira Weiny <ira.weiny@...el.com>, lsf-pc@...ts.linux-foundation.org,
        linux-rdma@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Doug Ledford <dledford@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Wed, Feb 06, 2019 at 10:50:00AM +0100, Jan Kara wrote:

> MM/FS asks for lease to be revoked. The revoke handler agrees with the
> other side on cancelling RDMA or whatever and drops the page pins. 

This takes a trip through userspace since the communication protocol
is entirely managed in userspace.

Most existing communication protocols don't have a 'cancel operation'.

> Now I understand there can be HW / communication failures etc. in
> which case the driver could either block waiting or make sure future
> IO will fail and drop the pins. 

We can always rip things away from the userspace.. However..

> But under normal conditions there should be a way to revoke the
> access. And if the HW/driver cannot support this, then don't let it
> anywhere near DAX filesystem.

I think the general observation is that people who want to do DAX &
RDMA want it to actually work, without data corruption, random process
kills or random communication failures.

Really, few users would actually want to run in a system where revoke
can be triggered.

So.. how can the FS/MM side provide a guarantee to the user that
revoke won't happen under a certain system design?

Jason