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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BBD5BC3DA6D for ; Tue, 20 May 2025 14:04:02 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uHNZN-00074o-SP; Tue, 20 May 2025 10:03:49 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uHNZK-00070W-W9 for qemu-devel@nongnu.org; Tue, 20 May 2025 10:03:47 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uHNZJ-0006oq-AA for qemu-devel@nongnu.org; Tue, 20 May 2025 10:03:46 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1747749824; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=n7ixXnJsvvTJitdRsTL9ixhhTWxyWh8J4K6H4I99UJk=; b=V5Y9zKOSlXYo89S5OvytWx8ZJ84jOvKd6FR82OX8+2FXAaFhUGR1c3t/hXYo9DDN9BjEV6 AgiTmqvLXv2hV+1KWymynWL1HxYnAxFz6x5c1XJz2pLXYBqmqm7hREt3Y8xoIOcvAzH3UZ jZ8CEtQS7BWnxUeovxnzXmR1qEmGxOo= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-548-eoj-uFIcPq-kJacP4mt_wA-1; Tue, 20 May 2025 10:03:42 -0400 X-MC-Unique: eoj-uFIcPq-kJacP4mt_wA-1 X-Mimecast-MFC-AGG-ID: eoj-uFIcPq-kJacP4mt_wA_1747749821 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4055718001EA; Tue, 20 May 2025 14:03:41 +0000 (UTC) Received: from localhost (unknown [10.2.16.246]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id B07CD30001AA; Tue, 20 May 2025 14:03:40 +0000 (UTC) Date: Tue, 20 May 2025 10:03:39 -0400 From: Stefan Hajnoczi To: Kevin Wolf Cc: qemu-block@nongnu.org, hreitz@redhat.com, pbonzini@redhat.com, bmarzins@redhat.com, qemu-devel@nongnu.org Subject: Re: [PATCH] file-posix: Probe paths and retry SG_IO on potential path errors Message-ID: <20250520140339.GC82528@fedora> References: <20250513113730.37404-1-kwolf@redhat.com> <20250513135148.GB227327@fedora> <20250515140142.GA333399@fedora> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="m/GStFk1LjxzomWv" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.487, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --m/GStFk1LjxzomWv Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, May 15, 2025 at 05:02:46PM +0200, Kevin Wolf wrote: > Am 15.05.2025 um 16:01 hat Stefan Hajnoczi geschrieben: > > On Thu, May 15, 2025 at 10:15:53AM +0200, Kevin Wolf wrote: > > > Am 13.05.2025 um 15:51 hat Stefan Hajnoczi geschrieben: > > > > On Tue, May 13, 2025 at 01:37:30PM +0200, Kevin Wolf wrote: > > > > > When scsi-block is used on a host multipath device, it runs into = the > > > > > problem that the kernel dm-mpath doesn't know anything about SCSI= or > > > > > SG_IO and therefore can't decide if a SG_IO request returned an e= rror > > > > > and needs to be retried on a different path. Instead of getting w= orking > > > > > failover, an error is returned to scsi-block and handled accordin= g to > > > > > the configured error policy. Obviously, this is not what users wa= nt, > > > > > they want working failover. > > > > >=20 > > > > > QEMU can parse the SG_IO result and determine whether this could = have > > > > > been a path error, but just retrying the same request could just = send it > > > > > to the same failing path again and result in the same error. > > > > >=20 > > > > > With a kernel that supports the DM_MPATH_PROBE_PATHS ioctl on dm-= mpath > > > > > block devices (queued in the device mapper tree for Linux 6.16), = we can > > > > > tell the kernel to probe all paths and tell us if any usable paths > > > > > remained. If so, we can now retry the SG_IO ioctl and expect it t= o be > > > > > sent to a working path. > > > > >=20 > > > > > Signed-off-by: Kevin Wolf > > > > > --- > > > > > block/file-posix.c | 82 ++++++++++++++++++++++++++++++++++++++++= +++++- > > > > > 1 file changed, 81 insertions(+), 1 deletion(-) > > > >=20 > > > > Maybe the probability of retry success would be higher with a delay= so > > > > that intermittent issues have time to resolve themselves. Either wa= y, > > > > the patch looks good. > > >=20 > > > I don't think adding a delay here would be helpful. The point of > > > multipath isn't that you wait until a bad path comes back, but that y= ou > > > just switch to a different path until it is restored. > >=20 > > That's not what this loop does. DM_MPATH_PROBE_PATHS probes all paths > > and fails when no paths are available. The delay would only apply in the > > case when there are no paths available. > >=20 > > If the point is not to wait until some path comes back, then why loop at > > all? >=20 > DM_MPATH_PROBE_PATHS can only send I/O to paths in the active path > group, so it doesn't fail over to different path groups. If there are no > usable paths left in the current path group, but there are some in > another one, then the ioctl returns 0 and the next SG_IO would switch to > a different path group, which may or may not succeed. If it fails, we > have to probe the paths in that group, too. This wasn't obvious to me, can that be emphasized in the code via naming or comments? About retrying up to 5 times: is the assumption that there will be 5 or fewer path groups? Stefan --m/GStFk1LjxzomWv Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmgsi7sACgkQnKSrs4Gr c8geZAgAytxvZIoiE30pEi6j2+qs50CQluSTjteJIn4lTjQbx6LFZca7GlmxEVWl 3VbC/dJvYRoCadfuXUUtbY4QqAPyauSygJBm1ROn4B6XXxJVrfv3fU6OxKAvshaP dBHq0l75G5Ib7FjpceC4nuFRV0vYt+UBwZyIxFqQETYB+C7DjKKp3V0Xq7LKA3S7 qJd7fcRWp1fT89TEauM5CfC9VuTLne4LGHCyzrzYcIMNCWPlTS4/grWyfT/wepnl 9K2vtWA875IhK9qt3THr59cHGEiaTITSzTelzmr07BfXCK+GpeRarQc0gm5Vx0nS 2wxCarsn9RTxfsZ64lqkvsVg5Tjqxw== =zb45 -----END PGP SIGNATURE----- --m/GStFk1LjxzomWv--