From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 675C4C433EF for ; Wed, 13 Jul 2022 08:04:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235043AbiGMIEl (ORCPT ); Wed, 13 Jul 2022 04:04:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35148 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235026AbiGMIEj (ORCPT ); Wed, 13 Jul 2022 04:04:39 -0400 Received: from mail-wm1-f46.google.com (mail-wm1-f46.google.com [209.85.128.46]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 547F8E5DC2; Wed, 13 Jul 2022 01:04:35 -0700 (PDT) Received: by mail-wm1-f46.google.com with SMTP id p27-20020a05600c1d9b00b003a2f36054d0so707161wms.4; Wed, 13 Jul 2022 01:04:35 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:in-reply-to :content-transfer-encoding; bh=IMbD90s21HdnddiL42xIyv27c0/QtqF4u8JsID8zNs0=; b=kCtcMtyKggjmPg2K17SKzaHN7Qw2OpNRYLnsafZdMWzloxrKm2i2o3ZoM7NHnarNx6 cgQk4uqcZP85SOvHO9th9QVImxLKs37NMmpJflFUIfO1t53+efmMFmj1k0wnBrVr7NB4 ZnkzO9iLaj7R6iTX7lZHfkKXZ213K5ZnSVF80wR1SlhHhdbi8qNRsNdbv+6pLEnkfMX4 /YGnn8dhygDpguDVC0rrj9BOi+1YzfI5pU+gJBuWSz7jm3F6Q45/4PPxbfgnzLaS7tYT 1vwn+30W9zBdjiAgFZ9hJRbtOBr5mpOAR04QP8hJJP8ucZw8fAEPOalqcwl6dBjODIVa jt0g== X-Gm-Message-State: AJIora94gVnGINfKCR1ZlaPI58FKZrnOXV+bO3oW72b7speyLHJA+Nrx b3CZBIovbjv06sKKxsTCWc4= X-Google-Smtp-Source: AGRyM1vwsh7bKCpn2lTmbIQEoG/5T6EX7m8IACBWbztC+wbdRF/oK3yyKEfzXUu/Co8455P2m9GISw== X-Received: by 2002:a05:600c:3545:b0:3a2:f3e3:c382 with SMTP id i5-20020a05600c354500b003a2f3e3c382mr2774942wmq.142.1657699473799; Wed, 13 Jul 2022 01:04:33 -0700 (PDT) Received: from [192.168.64.180] (bzq-219-42-90.isdn.bezeqint.net. [62.219.42.90]) by smtp.gmail.com with ESMTPSA id n187-20020a1c27c4000000b003a199ed4f44sm1566011wmn.27.2022.07.13.01.04.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Jul 2022 01:04:33 -0700 (PDT) Message-ID: <24f0a3e6-aa53-8c69-71b7-d66289a63eae@grimberg.me> Date: Wed, 13 Jul 2022 11:04:31 +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> From: Sagi Grimberg In-Reply-To: <20220713053633.GA13135@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 think the difference from scsi-generic and controller nvme passthru is >> that this is a mpath device node (or mpath chardev). This is why I think >> that users would expect that it would have equivalent multipath >> capabilities (i.e. failover). > > How is that different from /dev/sg? /dev/sg it is not a multipath device. 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... >> In general, I think that uring passthru as an alternative I/O interface >> and as such needs to be able to failover. If this is not expected from >> the interface, then why are we exposing a chardev for the mpath device >> node? why not only the bottom namespaces? > > The failover will happen when you retry, but we leave that retry to > userspace. There even is the uevent to tell userspace when a new > path is up. 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 basically needs to understand everything including indirectly path selection for the I/O. IMO it is cleaner for the user to just have the bottom devices and do the path selection directly.