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[62.219.42.90]) by smtp.gmail.com with ESMTPSA id q6-20020a1cf306000000b003a2e92edeccsm2058462wmq.46.2022.07.13.05.43.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Jul 2022 05:43:42 -0700 (PDT) Message-ID: <7c7a093c-4103-b67d-c145-9d84aaae835e@grimberg.me> Date: Wed, 13 Jul 2022 15:43:39 +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: Hannes Reinecke , 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> <96f47d9b-fbfc-80da-4c38-f46986f14a43@suse.de> From: Sagi Grimberg In-Reply-To: <96f47d9b-fbfc-80da-4c38-f46986f14a43@suse.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 7/13/22 14:49, Hannes Reinecke wrote: > On 7/13/22 13:00, Sagi Grimberg wrote: >> >>>> 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... >> > I did; 'twas me who was involved in the initial customer issue leading > up to that. > > Amongst all the other issue we've found the prime problem with SG_IO is > that it needs to be directed to the 'active' path. > For the device-mapper has a distinct callout (dm_prepare_ioctl), which > essentially returns the current active path device. And then the > device-mapper core issues the command on that active path. > > All nice and good, _unless_ that command triggers an error. > Normally it'd be intercepted by the dm-multipath end_io handler, and > would set the path to offline. > But as ioctls do not use the normal I/O path the end_io handler is never > called, and further SG_IO calls are happily routed down the failed path. > > And the customer had to use SG_IO (or, in qemu-speak, LUN passthrough) > as his application/filesystem makes heavy use of persistent reservations. How did this conclude Hannes?