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 03F25C2D0E4 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 9B8FC2083E for ; Tue, 17 Nov 2020 07:57:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726249AbgKQH43 (ORCPT ); Tue, 17 Nov 2020 02:56:29 -0500 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:44135 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726315AbgKQH43 (ORCPT ); Tue, 17 Nov 2020 02:56:29 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01424;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0UFgXvU5_1605599785; Received: from localhost(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UFgXvU5_1605599785) by smtp.aliyun-inc.com(127.0.0.1); Tue, 17 Nov 2020 15:56:25 +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 1/2] block: disable iopoll for split bio Date: Tue, 17 Nov 2020 15:56:24 +0800 Message-Id: <20201117075625.46118-2-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 iopoll is initially for small size, latency sensitive IO. It doesn't work well for big IO, especially when it needs to be split to multiple bios. In this case, the returned cookie of __submit_bio_noacct_mq() is indeed the cookie of the last split bio. The completion of *this* last split bio done by iopoll doesn't mean the whole original bio has completed. Callers of iopoll still need to wait for completion of other split bios. Besides bio splitting may cause more trouble for iopoll which isn't supposed to be used in case of big IO. iopoll for split bio may cause potential race if CPU migration happens during bio submission. Since the returned cookie is that of the last split bio, polling on the corresponding hardware queue doesn't help complete other split bios, if these split bios are enqueued into different hardware queues. Since interrupts are disabled for polling queues, the completion of these other split bios depends on timeout mechanism, thus causing a potential hang. iopoll for split bio may also cause hang for sync polling. Currently both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling in direct IO routine. These routines will submit bio without REQ_NOWAIT flag set, and then start sync polling in current process context. The process may hang in blk_mq_get_tag() if the submitted bio has to be split into multiple bios and can rapidly exhaust the queue depth. The process are waiting for the completion of the previously allocated requests, which should be reaped by the following polling, and thus causing a deadlock. To avoid these subtle trouble described above, just disable iopoll for split bio. Suggested-by: Ming Lei Signed-off-by: Jeffle Xu --- block/blk-merge.c | 7 +++++++ block/blk-mq.c | 6 ++++-- 2 files changed, 11 insertions(+), 2 deletions(-) diff --git a/block/blk-merge.c b/block/blk-merge.c index bcf5e4580603..53ad781917a2 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -279,6 +279,13 @@ static struct bio *blk_bio_segment_split(struct request_queue *q, return NULL; split: *segs = nsegs; + + /* + * bio splitting may cause subtle trouble such as hang when doing iopoll, + * not to mention iopoll isn't supposed to be used in case of big IO. + */ + bio->bi_opf &= ~REQ_HIPRI; + return bio_split(bio, sectors, GFP_NOIO, bs); } diff --git a/block/blk-mq.c b/block/blk-mq.c index 55bcee5dc032..6d10652a7ed0 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -3853,11 +3853,13 @@ int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin) !test_bit(QUEUE_FLAG_POLL, &q->queue_flags)) return 0; + hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)]; + if (hctx->type != HCTX_TYPE_POLL) + return 0; + if (current->plug) blk_flush_plug_list(current->plug, false); - hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)]; - /* * If we sleep, have the caller restart the poll loop to reset * the state. Like for the other success return cases, the -- 2.27.0