Re: [PATCH 14/14] xfs: use bounce buffering direct I/O when the device requires stable pages

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jan 19, 2026 at 08:44:21AM +0100, Christoph Hellwig wrote:
> Fix direct I/O on devices that require stable pages by asking iomap
> to bounce buffer.  To support this, ioends are used for direct reads
> in this case to provide a user context for copying data back from the
> bounce buffer.
> 
> This fixes qemu when used on devices using T10 protection information
> and probably other cases like iSCSI using data digests.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Ahaha, I forgot in the last round that s_dio_done_wq is not at all the
place for doing bio completions.
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_aops.c |  8 ++++++--
>  fs/xfs/xfs_file.c | 41 ++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 44 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 56a544638491..c3c1e149fff4 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -103,7 +103,7 @@ xfs_ioend_put_open_zones(
>   * IO write completion.
>   */
>  STATIC void
> -xfs_end_ioend(
> +xfs_end_ioend_write(
>  	struct iomap_ioend	*ioend)
>  {
>  	struct xfs_inode	*ip = XFS_I(ioend->io_inode);
> @@ -202,7 +202,11 @@ xfs_end_io(
>  			io_list))) {
>  		list_del_init(&ioend->io_list);
>  		iomap_ioend_try_merge(ioend, &tmp);
> -		xfs_end_ioend(ioend);
> +		if (bio_op(&ioend->io_bio) == REQ_OP_READ)
> +			iomap_finish_ioends(ioend,
> +				blk_status_to_errno(ioend->io_bio.bi_status));
> +		else
> +			xfs_end_ioend_write(ioend);
>  		cond_resched();
>  	}
>  }
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 7874cf745af3..f6cc63dcf961 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -224,12 +224,34 @@ xfs_ilock_iocb_for_write(
>  	return 0;
>  }
>  
> +/*
> + * Bounce buffering dio reads need a user context to copy back the data.
> + * Use an ioend to provide that.
> + */
> +static void
> +xfs_dio_read_bounce_submit_io(
> +	const struct iomap_iter	*iter,
> +	struct bio		*bio,
> +	loff_t			file_offset)
> +{
> +	iomap_init_ioend(iter->inode, bio, file_offset, IOMAP_IOEND_DIRECT);
> +	bio->bi_end_io = xfs_end_bio;
> +	submit_bio(bio);
> +}
> +
> +static const struct iomap_dio_ops xfs_dio_read_bounce_ops = {
> +	.submit_io	= xfs_dio_read_bounce_submit_io,
> +	.bio_set	= &iomap_ioend_bioset,
> +};
> +
>  STATIC ssize_t
>  xfs_file_dio_read(
>  	struct kiocb		*iocb,
>  	struct iov_iter		*to)
>  {
>  	struct xfs_inode	*ip = XFS_I(file_inode(iocb->ki_filp));
> +	unsigned int		dio_flags = 0;
> +	const struct iomap_dio_ops *dio_ops = NULL;
>  	ssize_t			ret;
>  
>  	trace_xfs_file_direct_read(iocb, to);
> @@ -242,7 +264,12 @@ xfs_file_dio_read(
>  	ret = xfs_ilock_iocb(iocb, XFS_IOLOCK_SHARED);
>  	if (ret)
>  		return ret;
> -	ret = iomap_dio_rw(iocb, to, &xfs_read_iomap_ops, NULL, 0, NULL, 0);
> +	if (mapping_stable_writes(iocb->ki_filp->f_mapping)) {
> +		dio_ops = &xfs_dio_read_bounce_ops;
> +		dio_flags |= IOMAP_DIO_BOUNCE;
> +	}
> +	ret = iomap_dio_rw(iocb, to, &xfs_read_iomap_ops, dio_ops, dio_flags,
> +			NULL, 0);
>  	xfs_iunlock(ip, XFS_IOLOCK_SHARED);
>  
>  	return ret;
> @@ -703,6 +730,8 @@ xfs_file_dio_write_aligned(
>  		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
>  		iolock = XFS_IOLOCK_SHARED;
>  	}
> +	if (mapping_stable_writes(iocb->ki_filp->f_mapping))
> +		dio_flags |= IOMAP_DIO_BOUNCE;
>  	trace_xfs_file_direct_write(iocb, from);
>  	ret = iomap_dio_rw(iocb, from, ops, dops, dio_flags, ac, 0);
>  out_unlock:
> @@ -750,6 +779,7 @@ xfs_file_dio_write_atomic(
>  {
>  	unsigned int		iolock = XFS_IOLOCK_SHARED;
>  	ssize_t			ret, ocount = iov_iter_count(from);
> +	unsigned int		dio_flags = 0;
>  	const struct iomap_ops	*dops;
>  
>  	/*
> @@ -777,8 +807,10 @@ xfs_file_dio_write_atomic(
>  	}
>  
>  	trace_xfs_file_direct_write(iocb, from);
> -	ret = iomap_dio_rw(iocb, from, dops, &xfs_dio_write_ops,
> -			0, NULL, 0);
> +	if (mapping_stable_writes(iocb->ki_filp->f_mapping))
> +		dio_flags |= IOMAP_DIO_BOUNCE;
> +	ret = iomap_dio_rw(iocb, from, dops, &xfs_dio_write_ops, dio_flags,
> +			NULL, 0);
>  
>  	/*
>  	 * The retry mechanism is based on the ->iomap_begin method returning
> @@ -867,6 +899,9 @@ xfs_file_dio_write_unaligned(
>  	if (flags & IOMAP_DIO_FORCE_WAIT)
>  		inode_dio_wait(VFS_I(ip));
>  
> +	if (mapping_stable_writes(iocb->ki_filp->f_mapping))
> +		flags |= IOMAP_DIO_BOUNCE;
> +
>  	trace_xfs_file_direct_write(iocb, from);
>  	ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
>  			   &xfs_dio_write_ops, flags, NULL, 0);
> -- 
> 2.47.3
> 
>