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 X-Spam-Level: X-Spam-Status: No, score=-9.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B0D02C433DF for ; Fri, 16 Oct 2020 10:26:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 42DB02084C for ; Fri, 16 Oct 2020 10:26:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="KEz91sOJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2395281AbgJPK0q (ORCPT ); Fri, 16 Oct 2020 06:26:46 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:58089 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2395282AbgJPK0q (ORCPT ); Fri, 16 Oct 2020 06:26:46 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1602844004; 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=1C4rOLd47+tSC7UT866Mf329X/wl57JsIP5mIOWf2N4=; b=KEz91sOJz5Off9QqCvgo+i0cncfuNIXqKRG5JsbcUBEPvJFtzZ+CKzMYk8a8O4lxCSWc8J 6MDU8Dx4jgLc5Das+R/lcYSQilZ1NWqkzpUpXAuboxY/DGJ+bBGBKPZaTZJbZcK8fgO+af TOReZove19kohxWZ9l5iS0vR3AmUeaE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-165-beDgsN6lNA-zUZx3z1E-fA-1; Fri, 16 Oct 2020 06:26:40 -0400 X-MC-Unique: beDgsN6lNA-zUZx3z1E-fA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4D9D3ADC26; Fri, 16 Oct 2020 10:26:38 +0000 (UTC) Received: from T590 (ovpn-12-93.pek2.redhat.com [10.72.12.93]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 518D176649; Fri, 16 Oct 2020 10:26:29 +0000 (UTC) Date: Fri, 16 Oct 2020 18:26:25 +0800 From: Ming Lei To: Jeffle Xu Cc: axboe@kernel.dk, hch@infradead.org, viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, joseph.qi@linux.alibaba.com, xiaoguang.wang@linux.alibaba.com Subject: Re: [PATCH v3 2/2] block,iomap: disable iopoll when split needed Message-ID: <20201016102625.GA1218835@T590> References: <20201016091851.93728-1-jefflexu@linux.alibaba.com> <20201016091851.93728-3-jefflexu@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201016091851.93728-3-jefflexu@linux.alibaba.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Oct 16, 2020 at 05:18:51PM +0800, Jeffle Xu wrote: > Both blkdev fs and iomap-based fs (ext4, xfs, etc.) currently support > sync iopoll. One single bio can contain at most BIO_MAX_PAGES, i.e. 256 > bio_vec. If the input iov_iter contains more than 256 segments, then > one dio will be split into multiple bios, which may cause potential > deadlock for sync iopoll. > > When it comes to sync iopoll, the bio is submitted without REQ_NOWAIT > flag set and the process may hang in blk_mq_get_tag() if the dio needs > to be split into multiple bios and thus can rapidly exhausts the queue > depth. The process has to wait for the completion of the previously > allocated requests, which should be reaped by the following sync > polling, and thus causing a deadlock. > > In fact there's a subtle difference of handling of HIPRI IO between > blkdev fs and iomap-based fs, when dio need to be split into multiple > bios. blkdev fs will set REQ_HIPRI for only the last split bio, leaving > the previous bios queued into normal hardware queues, and not causing > the trouble described above. iomap-based fs will set REQ_HIPRI for all > split bios, and thus may cause the potential deadlock decribed above. > > Thus disable iopoll when one dio need to be split into multiple bios. > Though blkdev fs may not suffer this issue, still it may not make much > sense to iopoll for big IO, since iopoll is initially for small size, > latency sensitive IO. > > Signed-off-by: Jeffle Xu > --- > fs/block_dev.c | 7 +++++++ > fs/iomap/direct-io.c | 8 ++++++++ > 2 files changed, 15 insertions(+) > > diff --git a/fs/block_dev.c b/fs/block_dev.c > index 9e84b1928b94..1b56b39e35b5 100644 > --- a/fs/block_dev.c > +++ b/fs/block_dev.c > @@ -436,6 +436,13 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages) > break; > } > > + /* > + * The current dio need to be split into multiple bios here. > + * iopoll is initially for small size, latency sensitive IO, > + * and thus disable iopoll if split needed. > + */ > + iocb->ki_flags &= ~IOCB_HIPRI; > + Not sure if it is good to clear IOCB_HIPRI of iocb, since it is usually maintained by upper layer code(io_uring, aio, ...) and we shouldn't touch this flag here. > if (!dio->multi_bio) { > /* > * AIO needs an extra reference to ensure the dio > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c > index c1aafb2ab990..46668cceefd2 100644 > --- a/fs/iomap/direct-io.c > +++ b/fs/iomap/direct-io.c > @@ -308,6 +308,14 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length, > copied += n; > > nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES); > + /* > + * The current dio need to be split into multiple bios here. > + * iopoll is initially for small size, latency sensitive IO, > + * and thus disable iopoll if split needed. > + */ > + if (nr_pages) > + dio->iocb->ki_flags &= ~IOCB_HIPRI; Same concern as above. Thanks, Ming