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 75C50C433EF for ; Wed, 13 Jul 2022 11:50:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236084AbiGMLuB (ORCPT ); Wed, 13 Jul 2022 07:50:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42086 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231264AbiGMLuB (ORCPT ); Wed, 13 Jul 2022 07:50:01 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4BDEAE4778; Wed, 13 Jul 2022 04:50:00 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 04A5220076; Wed, 13 Jul 2022 11:49:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1657712999; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NfMyiG6I6kY9duJdum1vSBmQPs459FBBxkpB0eMmIG4=; b=MBiFCVEpkGBkXylj/DRftkyXS3dQHP175Q8BVAy2q/IIyUM9glNzGIHwRU2mQZ96irpV2K DlzRfYX433mqXk8g+jxRPb880qGKAPuOjxu+0D8ZioPWBSGrDlCy6UDCr4tSNU+qBfky2l 35Ut5Xx1KWYMRX9VevE+QLW5Fwtkbkk= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1657712999; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NfMyiG6I6kY9duJdum1vSBmQPs459FBBxkpB0eMmIG4=; b=pGNNYSoAwdU5qBAB/O0zEtiuqF8w8l5ur5tzVXP2y/lncNKQlKY6cQX3+vnO+oYLbajtP6 gRrtRfWmCLKbv/BQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id A901B13AAD; Wed, 13 Jul 2022 11:49:58 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 4/GJKGaxzmKsFgAAMHmgww (envelope-from ); Wed, 13 Jul 2022 11:49:58 +0000 Message-ID: <96f47d9b-fbfc-80da-4c38-f46986f14a43@suse.de> Date: Wed, 13 Jul 2022 13:49:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Content-Language: en-US To: Sagi Grimberg , 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> From: Hannes Reinecke Subject: Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands In-Reply-To: <772b461a-bc43-c229-906d-0e280091e17f@grimberg.me> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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. So yeah, nvme-multipathing should be okay here, as io_uring and normal I/O are using the same code paths, hence the above scenario really won't occur. Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg HRB 36809 (AG Nürnberg), GF: Felix Imendörffer