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[174.3.196.123]) by smtp.gmail.com with ESMTPSA id u186sm10338671pfu.51.2019.02.06.15.41.33 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 06 Feb 2019 15:41:33 -0800 (PST) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1grWou-0004D9-Hy; Wed, 06 Feb 2019 16:41:32 -0700 Date: Wed, 6 Feb 2019 16:41:32 -0700 From: Jason Gunthorpe To: Dan Williams Cc: Doug Ledford , Dave Chinner , Christopher Lameter , Matthew Wilcox , Jan Kara , Ira Weiny , lsf-pc@lists.linux-foundation.org, linux-rdma , Linux MM , Linux Kernel Mailing List , John Hubbard , Jerome Glisse , Michal Hocko Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA Message-ID: <20190206234132.GB15234@ziepe.ca> References: <20190206173114.GB12227@ziepe.ca> <20190206175233.GN21860@bombadil.infradead.org> <47820c4d696aee41225854071ec73373a273fd4a.camel@redhat.com> <01000168c43d594c-7979fcf8-b9c1-4bda-b29a-500efe001d66-000000@email.amazonses.com> <20190206210356.GZ6173@dastard> <20190206220828.GJ12227@ziepe.ca> <0c868bc615a60c44d618fb0183fcbe0c418c7c83.camel@redhat.com> <20190206232130.GK12227@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 03:30:27PM -0800, Dan Williams wrote: > On Wed, Feb 6, 2019 at 3:21 PM Jason Gunthorpe wrote: > > > > On Wed, Feb 06, 2019 at 02:44:45PM -0800, Dan Williams wrote: > > > > > > Do they need to stick with xfs? > > > > > > Can you clarify the motivation for that question? This problem exists > > > for any filesystem that implements an mmap that where the physical > > > page backing the mapping is identical to the physical storage location > > > for the file data. > > > > .. and needs to dynamicaly change that mapping. Which is not really > > something inherent to the general idea of a filesystem. A file system > > that had *strictly static* block assignments would work fine. > > > > Not all filesystem even implement hole punch. > > > > Not all filesystem implement reflink. > > > > ftruncate doesn't *have* to instantly return the free blocks to > > allocation pool. > > > > ie this is not a DAX & RDMA issue but a XFS & RDMA issue. > > > > Replacing XFS is probably not be reasonable, but I wonder if a XFS-- > > operating mode could exist that had enough features removed to be > > safe? > > You're describing the current situation, i.e. Linux already implements > this, it's called Device-DAX and some users of RDMA find it > insufficient. The choices are to continue to tell them "no", or say > "yes, but you need to submit to lease coordination". Device-DAX is not what I'm imagining when I say XFS--. I mean more like XFS with all features that require rellocation of blocks disabled. Forbidding hold punch, reflink, cow, etc, doesn't devolve back to device-dax. Jason