Re: [PATCH v3 3/3] xfs: reject max_atomic_write mount option for no reflink

[Darrick J. Wong]

On Thu, Jul 24, 2025 at 08:12:15AM +0000, John Garry wrote:
> If the FS has no reflink, then atomic writes greater than 1x block are not
> supported. As such, for no reflink it is pointless to accept setting
> max_atomic_write when it cannot be supported, so reject max_atomic_write
> mount option in this case.
> 
> It could be still possible to accept max_atomic_write option of size 1x
> block if HW atomics are supported, so check for this specifically.
> 
> Fixes: 4528b9052731 ("xfs: allow sysadmins to specify a maximum atomic write limit at mount time")
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>

/me wonders if "mkfs: allow users to configure the desired maximum
atomic write size" needs a similar filter?

Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>

--D

> ---
>  fs/xfs/xfs_mount.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 0b690bc119d7..1ec70f4e57b4 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -784,6 +784,25 @@ xfs_set_max_atomic_write_opt(
>  		return -EINVAL;
>  	}
>  
> +	if (xfs_has_reflink(mp))
> +		goto set_limit;
> +
> +	if (new_max_fsbs == 1) {
> +		if (mp->m_ddev_targp->bt_awu_max ||
> +		    (mp->m_rtdev_targp && mp->m_rtdev_targp->bt_awu_max)) {
> +		} else {
> +			xfs_warn(mp,
> + "cannot support atomic writes of size %lluk with no reflink or HW support",
> +				new_max_bytes >> 10);
> +			return -EINVAL;
> +		}
> +	} else {
> +		xfs_warn(mp,
> + "cannot support atomic writes of size %lluk with no reflink support",
> +				new_max_bytes >> 10);
> +		return -EINVAL;
> +	}
> +
>  set_limit:
>  	error = xfs_calc_atomic_write_reservation(mp, new_max_fsbs);
>  	if (error) {
> -- 
> 2.43.5
> 
>