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=-11.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 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 B8ED8C433E7 for ; Fri, 16 Oct 2020 11:02:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 54C33207F7 for ; Fri, 16 Oct 2020 11:02:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2394532AbgJPLCr (ORCPT ); Fri, 16 Oct 2020 07:02:47 -0400 Received: from out30-44.freemail.mail.aliyun.com ([115.124.30.44]:58108 "EHLO out30-44.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2395545AbgJPLCr (ORCPT ); Fri, 16 Oct 2020 07:02:47 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R351e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04394;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0UCC3oqc_1602846164; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UCC3oqc_1602846164) by smtp.aliyun-inc.com(127.0.0.1); Fri, 16 Oct 2020 19:02:45 +0800 Subject: Re: [PATCH v3 2/2] block,iomap: disable iopoll when split needed To: Ming Lei 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 References: <20201016091851.93728-1-jefflexu@linux.alibaba.com> <20201016091851.93728-3-jefflexu@linux.alibaba.com> <20201016102625.GA1218835@T590> From: JeffleXu Message-ID: Date: Fri, 16 Oct 2020 19:02:44 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.12.1 MIME-Version: 1.0 In-Reply-To: <20201016102625.GA1218835@T590> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 10/16/20 6:26 PM, Ming Lei wrote: > 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 we queue bios into the DEFAULT hardware queue, but leaving the corresponding kiocb->ki_flags's IOCB_HIPRI set (exactly what the first patch does), is this another inconsistency? Please consider the following code snippet from __blkdev_direct_IO() ``` for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); if (!READ_ONCE(dio->waiter)) break; if (!(iocb->ki_flags & IOCB_HIPRI) || !blk_poll(bdev_get_queue(bdev), qc, true)) blk_io_schedule(); } ``` The IOCB_HIPRI flag is still set in iocb->ki_flags, but the corresponding bios are queued into DEFAULT hardware queue since the first patch. blk_poll() is still called in this case. > >> 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