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[62.219.42.90]) by smtp.gmail.com with ESMTPSA id q6-20020a1cf306000000b003a2e92edeccsm1986453wmq.46.2022.07.13.05.16.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Jul 2022 05:16:38 -0700 (PDT) Message-ID: <156ed583-5150-d823-437a-de5b9ffcda99@grimberg.me> Date: Wed, 13 Jul 2022 15:16:36 +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> <772b461a-bc43-c229-906d-0e280091e17f@grimberg.me> <20220713112845.GA780@lst.de> From: Sagi Grimberg In-Reply-To: <20220713112845.GA780@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org >> 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). > > Yeah. But the point is that if we have a path failure, the kernel > will pick a new path next time anyway, both in dm-mpath and nvme-mpath. If such a path is available 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. > > I think they are very useful here. Users of passthrough interface > need to be able to retry anyway, even on non-multipath setups. And > a dumb retry will do the right thing. I think you are painting a simple picture while this is not the case necessarily. It is not a dumb retry, because the user needs to determine if an available path for this particular namespace exists or wait for one if it doesn't want to do a submit/fail constant loop. A passthru interface does not mean that the user by definition needs to understand multipathing, ana/ctrl/ns states/mappings, etc. The user may just want to be able issue vendor-specific commands to the device. If the user needs to understand multipathing by definition, he/she has zero use of a mpath passthru device if it doesn't retry IMO.