From patchwork Fri Oct 16 09:18:50 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jingbo Xu X-Patchwork-Id: 11841117 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id D8E3914B5 for ; Fri, 16 Oct 2020 09:18:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C0A5B21D81 for ; Fri, 16 Oct 2020 09:18:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2406128AbgJPJSz (ORCPT ); Fri, 16 Oct 2020 05:18:55 -0400 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:42019 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2405437AbgJPJSy (ORCPT ); Fri, 16 Oct 2020 05:18:54 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R201e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04420;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0UCBNGHr_1602839932; Received: from localhost(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UCBNGHr_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 1/2] block: disable iopoll for split bio Date: Fri, 16 Oct 2020 17:18:50 +0800 Message-Id: <20201016091851.93728-2-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 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@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 | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/block/blk-merge.c b/block/blk-merge.c index bcf5e4580603..924db7c428b4 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -279,6 +279,20 @@ static struct bio *blk_bio_segment_split(struct request_queue *q, return NULL; split: *segs = nsegs; + + /* + * bio splitting may cause more trouble for iopoll which isn't supposed + * to be used in case of big IO. + * 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. + */ + bio->bi_opf &= ~REQ_HIPRI; + return bio_split(bio, sectors, GFP_NOIO, bs); } From patchwork Fri Oct 16 09:18:51 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jingbo Xu X-Patchwork-Id: 11841119 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 630BC17E6 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 417F4212CC 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 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);