Re: [PATCH v4 2/2] block,iomap: disable iopoll when split needed

[Christoph Hellwig <hch@infradead.org>]

> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 933f234d5bec..396ac0f91a43 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -309,6 +309,16 @@ 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 needs to be split into multiple bios here.
> +		 * iopoll for split bio will cause subtle trouble such as
> +		 * hang when doing sync polling, while iopoll is initially
> +		 * for small size, latency sensitive IO. Thus disable iopoll
> +		 * if split needed.
> +		 */
> +		if (nr_pages)
> +			dio->iocb->ki_flags &= ~IOCB_HIPRI;

I think this is confusing two things.  One is that we don't handle
polling well when there are multiple bios.  For this I think we should
only call bio_set_polled when we know there is a single bio.  But it
has nothing to do with a bio being split, as we can't know that at this
level.