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[174.3.196.123]) by smtp.gmail.com with ESMTPSA id d11sm8242868pgi.25.2019.02.06.09.31.16 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 06 Feb 2019 09:31:16 -0800 (PST) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1grR2Y-0003x0-U1; Wed, 06 Feb 2019 10:31:14 -0700 Date: Wed, 6 Feb 2019 10:31:14 -0700 From: Jason Gunthorpe To: Jan Kara Cc: Ira Weiny , lsf-pc@lists.linux-foundation.org, linux-rdma@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, John Hubbard , Jerome Glisse , Dan Williams , Matthew Wilcox , Dave Chinner , Doug Ledford , Michal Hocko Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA Message-ID: <20190206173114.GB12227@ziepe.ca> References: <20190205175059.GB21617@iweiny-DESK2.sc.intel.com> <20190206095000.GA12006@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190206095000.GA12006@quack2.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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