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=-9.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 CF071C433E7 for ; Fri, 16 Oct 2020 12:39:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7284A21556 for ; Fri, 16 Oct 2020 12:39:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="S2ev1WZR" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2394725AbgJPMjo (ORCPT ); Fri, 16 Oct 2020 08:39:44 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:35557 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2395004AbgJPMjo (ORCPT ); Fri, 16 Oct 2020 08:39:44 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1602851982; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=C0uj9cetJrhx8j6PK21cc9c8R84HWVVjoVhboDa35KU=; b=S2ev1WZR8V1Bgbnq70uGYMwvnUFJnoXFiQbzdFE4ZLtGWfux8a+T8wgSWS0JS9gcMp3k6A dVKleobhtwCoFizDRaDcC2LV7qtd8jC8akOOwXGbodoh1SmNl8lkTDYXnj2K01u86TZI7R OpFtH9IGAL9xtFpjJTQ1SUjwfLgIXRg= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-529-QG-3ExoaMmqLlKUbD7Tj6w-1; Fri, 16 Oct 2020 08:39:41 -0400 X-MC-Unique: QG-3ExoaMmqLlKUbD7Tj6w-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id AEC1F18A0775; Fri, 16 Oct 2020 12:39:38 +0000 (UTC) Received: from T590 (ovpn-12-93.pek2.redhat.com [10.72.12.93]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8C9AA7667D; Fri, 16 Oct 2020 12:39:30 +0000 (UTC) Date: Fri, 16 Oct 2020 20:39:25 +0800 From: Ming Lei To: JeffleXu 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 Subject: Re: [PATCH v3 2/2] block,iomap: disable iopoll when split needed Message-ID: <20201016123925.GB1218835@T590> References: <20201016091851.93728-1-jefflexu@linux.alibaba.com> <20201016091851.93728-3-jefflexu@linux.alibaba.com> <20201016102625.GA1218835@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Oct 16, 2020 at 07:02:44PM +0800, JeffleXu wrote: > > 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? My question is that if it is good for this code to clear IOCB_HIPRI of iocb, given this is the 1st such usage. And does io_uring implementation expect the flag to be cleared by lower layer? > > 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. It may be handled in the following way: if (!((iocb->ki_flags & IOCB_HIPRI) && !dio->multi_bio) || !blk_poll(bdev_get_queue(bdev), qc, true)) blk_io_schedule(); BTW, even for single bio with IOCB_HIPRI, the single fs bio can still be splitted, and blk_poll() will be called too. Thanks, Ming