Linux-Block Archive on lore.kernel.org
 help / color / Atom feed
From: Ming Lei <ming.lei@redhat.com>
To: Jeffle Xu <jefflexu@linux.alibaba.com>
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
Date: Fri, 16 Oct 2020 18:26:25 +0800
Message-ID: <20201016102625.GA1218835@T590> (raw)
In-Reply-To: <20201016091851.93728-3-jefflexu@linux.alibaba.com>

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 <jefflexu@linux.alibaba.com>
> ---
>  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 (!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


  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-16  9:18 [PATCH v3 0/2] block, iomap: disable iopoll for split bio Jeffle Xu
2020-10-16  9:18 ` [PATCH v3 1/2] block: " Jeffle Xu
2020-10-16 12:51   ` Ming Lei
2020-10-16  9:18 ` [PATCH v3 2/2] block,iomap: disable iopoll when split needed Jeffle Xu
2020-10-16 10:26   ` Ming Lei [this message]
2020-10-16 11:02     ` JeffleXu
2020-10-16 12:39       ` Ming Lei
2020-10-16 13:30         ` JeffleXu

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201016102625.GA1218835@T590 \
    --to=ming.lei@redhat.com \
    --cc=axboe@kernel.dk \
    --cc=hch@infradead.org \
    --cc=jefflexu@linux.alibaba.com \
    --cc=joseph.qi@linux.alibaba.com \
    --cc=linux-block@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=viro@zeniv.linux.org.uk \
    --cc=xiaoguang.wang@linux.alibaba.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Block Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-block/0 linux-block/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-block linux-block/ https://lore.kernel.org/linux-block \
		linux-block@vger.kernel.org
	public-inbox-index linux-block

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-block


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git