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[62.219.42.90]) by smtp.gmail.com with ESMTPSA id l1-20020a5d6681000000b0021d6924b777sm10417359wru.115.2022.07.13.04.00.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Jul 2022 04:00:58 -0700 (PDT) Message-ID: <772b461a-bc43-c229-906d-0e280091e17f@grimberg.me> Date: Wed, 13 Jul 2022 14:00:56 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands Content-Language: en-US To: Christoph Hellwig Cc: Kanchan Joshi , kbusch@kernel.org, axboe@kernel.dk, io-uring@vger.kernel.org, linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, asml.silence@gmail.com, joshiiitr@gmail.com, anuj20.g@samsung.com, gost.dev@samsung.com References: <20220711110155.649153-1-joshi.k@samsung.com> <20220711110155.649153-5-joshi.k@samsung.com> <20220712065250.GA6574@lst.de> <436c8875-5a99-4328-80ac-6a5aef7f16f4@grimberg.me> <20220713053633.GA13135@lst.de> <24f0a3e6-aa53-8c69-71b7-d66289a63eae@grimberg.me> <20220713101235.GA27815@lst.de> From: Sagi Grimberg In-Reply-To: <20220713101235.GA27815@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org >> Maybe the solution is to just not expose a /dev/ng for the mpath device >> node, but only for bottom namespaces. Then it would be completely >> equivalent to scsi-generic devices. >> >> It just creates an unexpected mix of semantics of best-effort >> multipathing with just path selection, but no requeue/failover... > > Which is exactly the same semanics as SG_IO on the dm-mpath nodes. I view uring passthru somewhat as a different thing than sending SG_IO ioctls to dm-mpath. But it can be argued otherwise. BTW, the only consumer of it that I'm aware of commented that he expects dm-mpath to retry SG_IO when dm-mpath retry for SG_IO submission was attempted (https://www.spinics.net/lists/dm-devel/msg46924.html). From Paolo: "The problem is that userspace does not have a way to direct the command to a different path in the resubmission. It may not even have permission to issue DM_TABLE_STATUS, or to access the /dev nodes for the underlying paths, so without Martin's patches SG_IO on dm-mpath is basically unreliable by design." I didn't manage to track down any followup after that email though... >> If the user needs to do the retry, discover and understand namespace >> paths, ANA states, controller states, etc. Why does the user need a >> mpath chardev at all? > > The user needs to do that for all kinds of other resons anyway, > as we don't do any retries for passthrough at all. I still think that there is a problem with the existing semantics for passthru requests over mpath device nodes. Again, I think it will actually be cleaner not to expose passthru devices for mpath at all if we are not going to support retry/failover.