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

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Date:   Wed, 6 Feb 2019 09:52:33 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jan Kara <jack@...e.cz>, 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>,
        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 10:31:14AM -0700, Jason Gunthorpe wrote:
> 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?

Most of the cases we want revoke for are things like truncate().
Shouldn't happen with a sane system, but we're trying to avoid users
doing awful things like being able to DMA to pages that are now part of
a different file.