Re: [PATCH v6] block: disable iopoll for split bio

[Ming Lei <ming.lei@redhat.com>]

On Wed, Nov 25, 2020 at 02:41:47PM +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 <ming.lei@redhat.com>
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/bio.c               |  2 ++
>  block/blk-merge.c         | 12 ++++++++++++
>  block/blk-mq.c            |  3 +++
>  include/linux/blk_types.h |  1 +
>  4 files changed, 18 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index fa01bef35bb1..7f7ddc22a30d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>  	bio_set_flag(bio, BIO_CLONED);
>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>  		bio_set_flag(bio, BIO_THROTTLED);
> +	if (bio_flagged(bio_src, BIO_SPLIT))
> +		bio_set_flag(bio, BIO_SPLIT);
>  	bio->bi_opf = bio_src->bi_opf;
>  	bio->bi_ioprio = bio_src->bi_ioprio;
>  	bio->bi_write_hint = bio_src->bi_write_hint;
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index bcf5e4580603..a2890cebf99f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -279,6 +279,18 @@ 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 sync
> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
> +	 * big IO can be trival, disable iopoll when split needed. We need
> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
> +	 * only marked here.
> +	 */
> +	bio->bi_opf &= ~REQ_HIPRI;
> +	bio_set_flag(bio, BIO_SPLIT);

You may need to put the above into one helper, and call the helper for
other splitted cases(discard, write zero and write same) too.

thanks,
Ming