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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 589CCC56201 for ; Thu, 19 Nov 2020 17:52:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CA4BA246CA for ; Thu, 19 Nov 2020 17:52:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="hEO4IqYN" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727182AbgKSRwu (ORCPT ); Thu, 19 Nov 2020 12:52:50 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39010 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727171AbgKSRwu (ORCPT ); Thu, 19 Nov 2020 12:52:50 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1777DC0613CF; Thu, 19 Nov 2020 09:52:50 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=cQkJ8oOJbey+jhde6R1GtaMp6RE8pauahlmMPFtXjZs=; b=hEO4IqYNoc19Blikcdoe2QXwBn ybNPIxxCWJWZXd6iQlUNxZUHh79QDprUSf7QQ/mHJLvv1JWsY693pbYp/Pl4ddsj0vuaxUe4OzZFz vVXAV0aGpsPDxlRBTDE9GhOSRz9xs6wi3qFy4uLXgUe68l0rfPEkK8CKo2kLMBeEw6ds0Dv7CkXDU L5MOnJLTkC/YhEVHADMZKcaMsFfu89FCcPOedkE5dru2/soU3sp4CCVTCfer4o9BK6GiJ6CMhVa/V UzZ5x7zdlP1/5DaV3ZyRIr0ZneuehElgdb6dL0clD482W9RfVwxn1j3nt+N7wiGHJRpJmJ8AAylLF LrnX29Bw==; Received: from hch by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfo6k-0005fU-SJ; Thu, 19 Nov 2020 17:52:34 +0000 Date: Thu, 19 Nov 2020 17:52:34 +0000 From: Christoph Hellwig To: Jeffle Xu Cc: axboe@kernel.dk, hch@infradead.org, ming.lei@redhat.com, linux-block@vger.kernel.org, io-uring@vger.kernel.org, joseph.qi@linux.alibaba.com Subject: Re: [PATCH v4 1/2] block: disable iopoll for split bio Message-ID: <20201119175234.GA20944@infradead.org> References: <20201117075625.46118-1-jefflexu@linux.alibaba.com> <20201117075625.46118-2-jefflexu@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201117075625.46118-2-jefflexu@linux.alibaba.com> X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Tue, Nov 17, 2020 at 03:56:24PM +0800, Jeffle Xu wrote: > 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, Please capitalize the first character of a multi-line comments. Also this adds an overly long line. > + hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)]; > + if (hctx->type != HCTX_TYPE_POLL) > + return 0; I think this is good as a sanity check, but shouldn't we be able to avoid even hitting this patch if we ensure that BLK_QC_T_NONE is returned after a bio is split?