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 1BA4DC433F5 for ; Tue, 10 May 2022 02:58:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234932AbiEJDCM (ORCPT ); Mon, 9 May 2022 23:02:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57138 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232464AbiEJDCM (ORCPT ); Mon, 9 May 2022 23:02:12 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 81F92286FC0 for ; Mon, 9 May 2022 19:58:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1652151494; 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=VfThjiwVEwPrOow8FSqurh9iv72UgtJ9TH9+GAaixz4=; b=O6tgTuGon8zFLifRbeHc8Vf/CbImwP+z6RHWUdPr36zgqPs27z9oGhyjr8zaO8rQTDwi0o WCL6ghuFBnJ7Xe/iXilyleISHpryr1jwdWezBrF9rzg+/nFEzvvGgtfvkyYyy5y6ONlloA jHV2uJoLjaDV5KkfyaboQ0C7KthyNBw= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-187-ANsr0xFJNlum0wru0cPDWQ-1; Mon, 09 May 2022 22:58:13 -0400 X-MC-Unique: ANsr0xFJNlum0wru0cPDWQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E0B8129AB41E; Tue, 10 May 2022 02:58:12 +0000 (UTC) Received: from T590 (ovpn-8-16.pek2.redhat.com [10.72.8.16]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C15B940D2829; Tue, 10 May 2022 02:58:07 +0000 (UTC) Date: Tue, 10 May 2022 10:58:01 +0800 From: Ming Lei To: Jens Axboe Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, io-uring@vger.kernel.org, Gabriel Krisman Bertazi , ZiyangZhang , Xiaoguang Wang Subject: Re: [RFC PATCH] ubd: add io_uring based userspace block driver Message-ID: References: <20220509092312.254354-1-ming.lei@redhat.com> <9c833e12-fd09-fe7d-d4f2-e916c6ce4524@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9c833e12-fd09-fe7d-d4f2-e916c6ce4524@kernel.dk> X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Mon, May 09, 2022 at 10:09:10AM -0600, Jens Axboe wrote: > On 5/9/22 3:23 AM, Ming Lei wrote: > > This is the driver part of userspace block driver(ubd driver), the other > > part is userspace daemon part(ubdsrv)[1]. > > > > The two parts communicate by io_uring's IORING_OP_URING_CMD with one > > shared cmd buffer for storing io command, and the buffer is read only for > > ubdsrv, each io command is indexed by io request tag directly, and > > is written by ubd driver. > > > > For example, when one READ io request is submitted to ubd block driver, ubd > > driver stores the io command into cmd buffer first, then completes one > > IORING_OP_URING_CMD for notifying ubdsrv, and the URING_CMD is issued to > > ubd driver beforehand by ubdsrv for getting notification of any new io request, > > and each URING_CMD is associated with one io request by tag. > > > > After ubdsrv gets the io command, it translates and handles the ubd io > > request, such as, for the ubd-loop target, ubdsrv translates the request > > into same request on another file or disk, like the kernel loop block > > driver. In ubdsrv's implementation, the io is still handled by io_uring, > > and share same ring with IORING_OP_URING_CMD command. When the target io > > request is done, the same IORING_OP_URING_CMD is issued to ubd driver for > > both committing io request result and getting future notification of new > > io request. > > > > Another thing done by ubd driver is to copy data between kernel io > > request and ubdsrv's io buffer: > > > > 1) before ubsrv handles WRITE request, copy the request's data into > > ubdsrv's userspace io buffer, so that ubdsrv can handle the write > > request > > > > 2) after ubsrv handles READ request, copy ubdsrv's userspace io buffer > > into this READ request, then ubd driver can complete the READ request > > > > Zero copy may be switched if mm is ready to support it. > > > > ubd driver doesn't handle any logic of the specific user space driver, > > so it should be small/simple enough. > > This is pretty interesting! Just one small thing I noticed, since you > want to make sure batching is Good Enough: > > > +static blk_status_t ubd_queue_rq(struct blk_mq_hw_ctx *hctx, > > + const struct blk_mq_queue_data *bd) > > +{ > > + struct ubd_queue *ubq = hctx->driver_data; > > + struct request *rq = bd->rq; > > + struct ubd_io *io = &ubq->ios[rq->tag]; > > + struct ubd_rq_data *data = blk_mq_rq_to_pdu(rq); > > + blk_status_t res; > > + > > + if (ubq->aborted) > > + return BLK_STS_IOERR; > > + > > + /* this io cmd slot isn't active, so have to fail this io */ > > + if (WARN_ON_ONCE(!(io->flags & UBD_IO_FLAG_ACTIVE))) > > + return BLK_STS_IOERR; > > + > > + /* fill iod to slot in io cmd buffer */ > > + res = ubd_setup_iod(ubq, rq); > > + if (res != BLK_STS_OK) > > + return BLK_STS_IOERR; > > + > > + blk_mq_start_request(bd->rq); > > + > > + /* mark this cmd owned by ubdsrv */ > > + io->flags |= UBD_IO_FLAG_OWNED_BY_SRV; > > + > > + /* > > + * clear ACTIVE since we are done with this sqe/cmd slot > > + * > > + * We can only accept io cmd in case of being not active. > > + */ > > + io->flags &= ~UBD_IO_FLAG_ACTIVE; > > + > > + /* > > + * run data copy in task work context for WRITE, and complete io_uring > > + * cmd there too. > > + * > > + * This way should improve batching, meantime pinning pages in current > > + * context is pretty fast. > > + */ > > + task_work_add(ubq->ubq_daemon, &data->work, TWA_SIGNAL); > > + > > + return BLK_STS_OK; > > +} > > It'd be better to use bd->last to indicate what kind of signaling you > need here. TWA_SIGNAL will force an immediate transition if the app is > running in userspace, which may not be what you want. Also see: > > https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.19/io_uring&id=e788be95a57a9bebe446878ce9bf2750f6fe4974 > > But regardless of signaling needed, you don't need it except if bd->last > is true. Would need a commit_rqs() as well, but that's trivial. Good point, I think we may add non-last request via task_work_add(TWA_NONE), and only notify via TWA_SIGNAL_NO_IPI for bd->last. > > More importantly, what prevents ubq->ubq_daemon from going away after > it's been assigned? I didn't look at the details, but is this relying on > io_uring being closed to cancel pending requests? That should work, but I think no way can prevent ubq->ubq_daemon from being killed by 'kill -9', even though ubdsrv has handled SIGTERM. That is why I suggest to add one service for removing all ubd devices before shutdown: https://github.com/ming1/ubdsrv/blob/devel/README All the commands of UBD_IO_FETCH_REQ or UBD_IO_COMMIT_AND_FETCH_REQ have been submitted to driver, I understand io_uring can't cancel them, please correct me if it is wrong. One solution I thought of is to use one watchdog to check if ubq->ubq_daemon is dead, then abort whole device if yes. Or any suggestion? > we need some way to ensure that ->ubq_daemon is always valid here. Good catch. get_task_struct() should be used for assigning ubq->ubq_daemon. thanks, Ming