From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0C0AF1EBA10 for ; Tue, 15 Oct 2024 12:11:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728994281; cv=none; b=rKdBbuHxe6OuBbiv5gwshWsRRiQh73YFyfz/OSNBxxDDHCEFi7DTmVVzB14h4I98NgL/MEW8etPwD6j3GGzjvrwgnYcM5hIpmPefFZrbZGoW9Mxt0WV/0FyfTVQdTPuWIEWLS3T0MXNqSvX0GyB5k1yyzYQDkRhLLPT5H4jUrLI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728994281; c=relaxed/simple; bh=V+4C3/dscvAkJcP+wPFslM1V6UlX9KAE3mMI9JhJ9IU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=irQnwpwEh32QDyTk/3mUrVWcXxN5ojiMiJeCYi62BAIWEqn4S5vENNJkRpROhcAbjGCsmEXt2vrXjB5zfESqv7zrUFM62ErjvwLdDFbdb0YbKvTJAfSVZuapRzEfut8ddFjITYqExuOt1mi7rwLdJ/VaKyRiN9HLjPM7u5yj9zM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=DqyiHuce; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="DqyiHuce" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1728994277; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=G3Pcz/dEOnqKX/cFViUwxujJai8oBAzKnB5CGzzdEx0=; b=DqyiHucewE8IJ9Swt67LtHlt8v7fZr2KzjVpF3XY8iUaG+Sw4kmRZFSauF8Pgq8QiAFz46 oHGXmi+b1XCWRcNVtjvz6oZytbGb+e+chha09sX7QDifgYHAgRXPJF8uQZ/blTYyJApB8M TIpPwF4cv4kr4S2gdmZuDgOJ+F6tmxU= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-205-hIfcxqtOOSO0A-W20qWf1g-1; Tue, 15 Oct 2024 08:11:14 -0400 X-MC-Unique: hIfcxqtOOSO0A-W20qWf1g-1 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B8ED51956083; Tue, 15 Oct 2024 12:11:13 +0000 (UTC) Received: from fedora (unknown [10.72.116.121]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9A2EA1956056; Tue, 15 Oct 2024 12:11:08 +0000 (UTC) Date: Tue, 15 Oct 2024 20:11:02 +0800 From: Ming Lei To: Kevin Wolf Cc: josef@toxicpanda.com, axboe@kernel.dk, linux-block@vger.kernel.org, nbd@other.debian.org, eblake@redhat.com, ming.lei@redhat.com Subject: Re: Kernel NBD client waits on wrong cookie, aborts connection Message-ID: References: Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On Tue, Oct 15, 2024 at 08:01:43PM +0800, Ming Lei wrote: > On Tue, Oct 15, 2024 at 6:22 PM Kevin Wolf wrote: > > > > Hi all, > > > > the other day I was running some benchmarks to compare different QEMU > > block exports, and one of the scenarios I was interested in was > > exporting NBD from qemu-storage-daemon over a unix socket and attaching > > it as a block device using the kernel NBD client. I would then run a VM > > on top of it and fio inside of it. > > > > Unfortunately, I couldn't get any numbers because the connection always > > aborted with messages like "Double reply on req ..." or "Unexpected > > reply ..." in the host kernel log. > > > > Yesterday I found some time to have a closer look why this is happening, > > and I think I have a rough understanding of what's going on now. Look at > > these trace events: > > > > qemu-img-51025 [005] ..... 19503.285423: nbd_header_sent: nbd transport event: request 000000002df03708, handle 0x0000150c0000005a > > [...] > > qemu-img-51025 [008] ..... 19503.285500: nbd_payload_sent: nbd transport event: request 000000002df03708, handle 0x0000150c0000005d > > [...] > > kworker/u49:1-47350 [004] ..... 19503.285514: nbd_header_received: nbd transport event: request 00000000b79e7443, handle 0x0000150c0000005a > > > > This is the same request, but the handle has changed between > > nbd_header_sent and nbd_payload_sent! I think this means that we hit one > > of the cases where the request is requeued, and then the next time it > > is executed with a different blk-mq tag, which is something the nbd > > driver doesn't seem to expect. > > > > Of course, since the cookie is transmitted in the header, the server > > replies with the original handle that contains the tag from the first > > call, while the kernel is only waiting for a handle with the new tag and > > is confused by the server response. > > > > I'm not sure yet which of the following options should be considered the > > real problem here, so I'm only describing the situation without trying > > to provide a patch: > > > > 1. Is it that blk-mq should always re-run the request with the same tag? > > I don't expect so, though in practice I was surprised to see that it > > happens quite often after nbd requeues a request that it actually > > does end up with the same cookie again. > > No. > > request->tag will change, but we may take ->internal_tag(sched) or > ->tag(none), which won't change. > > I guess was_interrupted() in nbd_send_cmd() is triggered, then the payload > is sent with a different tag. > > I will try to cook one patch soon. Please try the following patch: diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 2cafcf11ee8b..e3eb31c3ee75 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -682,3 +682,16 @@ u32 blk_mq_unique_tag(struct request *rq) (rq->tag & BLK_MQ_UNIQUE_TAG_MASK); } EXPORT_SYMBOL(blk_mq_unique_tag); + +/* + * Same with blk_mq_unique_tag, but one persistent tag is included in + * the request lifetime. + */ +u32 blk_mq_unique_static_tag(struct request *rq) +{ + u32 tag = rq->q->elevator ? rq->internal_tag : rq->tag; + + return (rq->mq_hctx->queue_num << BLK_MQ_UNIQUE_TAG_BITS) | + (tag & BLK_MQ_UNIQUE_TAG_MASK); +} +EXPORT_SYMBOL(blk_mq_unique_static_tag); diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c index b852050d8a96..cc522a2cb9fb 100644 --- a/drivers/block/nbd.c +++ b/drivers/block/nbd.c @@ -201,7 +201,7 @@ static void nbd_requeue_cmd(struct nbd_cmd *cmd) static u64 nbd_cmd_handle(struct nbd_cmd *cmd) { struct request *req = blk_mq_rq_from_pdu(cmd); - u32 tag = blk_mq_unique_tag(req); + u32 tag = blk_mq_unique_static_tag(req); u64 cookie = cmd->cmd_cookie; return (cookie << NBD_COOKIE_BITS) | tag; diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h index 4fecf46ef681..d6266759d62d 100644 --- a/include/linux/blk-mq.h +++ b/include/linux/blk-mq.h @@ -793,6 +793,7 @@ enum { }; u32 blk_mq_unique_tag(struct request *rq); +u32 blk_mq_unique_static_tag(struct request *rq); static inline u16 blk_mq_unique_tag_to_hwq(u32 unique_tag) { -- Ming