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Thu, 30 Jan 2025 14:54:05 -0800 (PST) Received: from Borg-518.local ([2603:8080:1500:3d89:ade3:d3:5a45:83b9]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-726617cdd7esm559916a34.21.2025.01.30.14.54.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jan 2025 14:54:04 -0800 (PST) From: John Groves X-Google-Original-From: John Groves Date: Thu, 30 Jan 2025 16:54:03 -0600 To: lsf-pc@lists.linux-foundation.org Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-cxl@vger.kernel.org, Dan Williams , Ira Weiny , Jonathan Cameron , Amir Goldstein , Miklos Szeredi , john@groves.net Subject: [LSF/MM/BPF TOPIC] Dax, memfd, guest_memfd, cxl, famfs - Is there redundancy here? Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 417D41A0004 X-Stat-Signature: ak4gpf1cp4mpg66g6cy5ygoxotm8txfr X-Rspam-User: X-HE-Tag: 1738277646-419728 X-HE-Meta: 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 NwCEgObF 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.107692, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: I have been hearing comments that there might be redundancy between dax and memfd - and have recently become aware of the guest_memfd work. A session where we discuss these related abstractions, their use cases, and whether there is redundancy seems like it would be useful. I come at this primarily as the author of famfs [1,2,3,4], which exposes disaggregated shared memory as a scale-out fs-dax file system on devdax memory (with no block backing store. Famfs is currently dependent on a dax instance for each memory device (or each tagged allocation, in the case of cxl DCDs (dynamic-capacity devices). DCDs create a "tag namespace" to memory/devdax devices (and tags are basically UUIDs). Famfs, similar to conventional file systems that live on block devices, uses the device abstraction of devdax to identify and access the backing memory for a file system. Much like block devices generally have recognizable superblocks at offset 0 (see lsblk etc.), sharable memory devices have UUIDs, and may also have superblocks. The device abstraction of tagged memory is a very useful property. I've been asked a number of times whether famfs could live on a memfd, and I currently think the answer is no - but I think we are at a point where these abstractions should be examined and discussed in context. Brief famfs status: At LSFMM '24 the consensus was that it should be ported into fuse. That work is getting close but not quite ready to post patches. Those should start to appear this spring. [1] https://github.com/cxl-micron-reskit/famfs [2] https://lwn.net/Articles/983105/ [3] https://lore.kernel.org/linux-cxl/cover.1708709155.git.john@groves.net/ [4] https://lore.kernel.org/linux-cxl/cover.1714409084.git.john@groves.net/ Cheers, John