Re: [PATCH v4 1/2] block: disable iopoll for split bio

[Christoph Hellwig <hch@infradead.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 <ming.lei@redhat.com>
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>  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?