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=-12.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_GIT autolearn=ham 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 857E2C2BD0C for ; Fri, 16 Oct 2020 09:18:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3F48020872 for ; Fri, 16 Oct 2020 09:18:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405448AbgJPJS4 (ORCPT ); Fri, 16 Oct 2020 05:18:56 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:60455 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2406076AbgJPJSy (ORCPT ); Fri, 16 Oct 2020 05:18:54 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R811e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04395;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0UCBA.FS_1602839932; Received: from localhost(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UCBA.FS_1602839932) by smtp.aliyun-inc.com(127.0.0.1); Fri, 16 Oct 2020 17:18:52 +0800 From: Jeffle Xu To: axboe@kernel.dk, hch@infradead.org, viro@zeniv.linux.org.uk Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, ming.lei@redhat.com, joseph.qi@linux.alibaba.com, xiaoguang.wang@linux.alibaba.com Subject: [PATCH v3 2/2] block,iomap: disable iopoll when split needed Date: Fri, 16 Oct 2020 17:18:51 +0800 Message-Id: <20201016091851.93728-3-jefflexu@linux.alibaba.com> X-Mailer: git-send-email 2.27.0 In-Reply-To: <20201016091851.93728-1-jefflexu@linux.alibaba.com> References: <20201016091851.93728-1-jefflexu@linux.alibaba.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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; + 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; + iomap_dio_submit_bio(dio, iomap, bio, pos); pos += n; } while (nr_pages); -- 2.27.0