Re: [PATCH 1/2] block: fail a short atomic bounce in bio_iov_iter_bounce_write()

[John Garry <john.garry@linux.dev>]

On 11/8/26 04:24, Tal Zussman wrote:
> bio_iov_iter_get_pages() fails a REQ_ATOMIC bio that doesn't cover the
> whole iterator, since an atomic write is submitted as a single bio, and
> submitting only part of it would tear the write. In contrast,
> bio_iov_iter_bounce_write() doesn't. If a folio allocation fails partway
> through, it returns a short bio, which for an atomic write triggers a
> WARN_ON_ONCE() in iomap_dio_bio_iter_one().
> 
> Make bio_iov_iter_bounce_write() also check that a REQ_ATOMIC bio covers
> the whole iterator, and clean up and return -EINVAL if it doesn't.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>

This looks reasonable, as atomic stable writes are implied to be 
supported from 3373503df025a ("xfs: use bounce buffering direct I/O when 
the device requires stable pages"). I am wondering how to test and prove 
this (change is good and maintained)?

> ---
>   block/bio.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index b0fcc69de653..61a3812520a2 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1523,6 +1523,16 @@ int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
>   		}
>   	}
>   
> +	/*
> +	 * An atomic write is submitted as a single bio, so it has to cover
> +	 * the whole iterator or it would be torn.
> +	 */
> +	if ((bio->bi_opf & REQ_ATOMIC) && iov_iter_count(iter)) {
> +		iov_iter_revert(iter, total_copied);
> +		bio_free_folios(bio);
> +		return -EINVAL;
> +	}
> +
>   	return 0;
>   }
>   EXPORT_SYMBOL_GPL(bio_iov_iter_bounce_write);
>