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 31850C5519F for ; Tue, 17 Nov 2020 07:57:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C88B3222E9 for ; Tue, 17 Nov 2020 07:57:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726815AbgKQH43 (ORCPT ); Tue, 17 Nov 2020 02:56:29 -0500 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:46271 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726136AbgKQH43 (ORCPT ); Tue, 17 Nov 2020 02:56:29 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R151e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=alimailimapcm10staff010182156082;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0UFgHDzY_1605599786; Received: from localhost(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UFgHDzY_1605599786) by smtp.aliyun-inc.com(127.0.0.1); Tue, 17 Nov 2020 15:56:26 +0800 From: Jeffle Xu To: axboe@kernel.dk, hch@infradead.org, ming.lei@redhat.com Cc: linux-block@vger.kernel.org, io-uring@vger.kernel.org, joseph.qi@linux.alibaba.com Subject: [PATCH v4 2/2] block,iomap: disable iopoll when split needed Date: Tue, 17 Nov 2020 15:56:25 +0800 Message-Id: <20201117075625.46118-3-jefflexu@linux.alibaba.com> X-Mailer: git-send-email 2.27.0 In-Reply-To: <20201117075625.46118-1-jefflexu@linux.alibaba.com> References: <20201117075625.46118-1-jefflexu@linux.alibaba.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@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 potential 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 described above. Noted that though the analysis described above, currently blkdev fs and iomap-based fs won't trigger this potential deadlock. Because only preadv2(2)/pwritev2(2) are capable of *sync* polling as only these two can set RWF_NOWAIT. Currently the maximum number of iovecs of one single preadv2(2)/pwritev2(2) call is UIO_MAXIOV, i.e. 1024, while the minimum queue depth is BLKDEV_MIN_RQ i.e. 4. That means one preadv2(2)/pwritev2(2) call can submit at most 4 bios, which will fill up the queue depth *exactly* and thus there's no deadlock in this case. However this constraint can be fragile. 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 | 9 +++++++++ fs/iomap/direct-io.c | 10 ++++++++++ 2 files changed, 19 insertions(+) diff --git a/fs/block_dev.c b/fs/block_dev.c index 9e84b1928b94..ed3f46e8fa91 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -436,6 +436,15 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages) break; } + /* + * The current dio needs to be split into multiple bios here. + * iopoll for split bio will cause subtle trouble such as + * hang when doing sync polling, while iopoll is initially + * for small size, latency sensitive IO. 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 933f234d5bec..396ac0f91a43 100644 --- a/fs/iomap/direct-io.c +++ b/fs/iomap/direct-io.c @@ -309,6 +309,16 @@ 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 needs to be split into multiple bios here. + * iopoll for split bio will cause subtle trouble such as + * hang when doing sync polling, while iopoll is initially + * for small size, latency sensitive IO. 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