xfs and DAX atomic writes changes

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

Posted by John Garry 6 months, 2 weeks ago

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@oracle.com>
---
 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

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

Posted by Darrick J. Wong 6 months, 2 weeks ago

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@oracle.com>

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

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

--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
> 
>

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

Posted by John Garry 6 months, 2 weeks ago

On 24/07/2025 17:32, Darrick J. Wong wrote:
> 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@oracle.com>
> /me wonders if "mkfs: allow users to configure the desired maximum
> atomic write size" needs a similar filter?
> 

Yeah, probably. But I am wondering if we should always require reflink 
for setting that max atomic mkfs option, and not have a special case of 
HW atomics available for 1x blocksize atomic writes.

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

cheers

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

Posted by Darrick J. Wong 6 months, 2 weeks ago

On Fri, Jul 25, 2025 at 09:39:42AM +0100, John Garry wrote:
> On 24/07/2025 17:32, Darrick J. Wong wrote:
> > 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@oracle.com>
> > /me wonders if "mkfs: allow users to configure the desired maximum
> > atomic write size" needs a similar filter?
> > 
> 
> Yeah, probably. But I am wondering if we should always require reflink for
> setting that max atomic mkfs option, and not have a special case of HW
> atomics available for 1x blocksize atomic writes.

I think that's reasonable for mkfs since reflink=1 has been the default
for quite a long while now.

--D

> > Reviewed-by: "Darrick J. Wong"<djwong@kernel.org>
> 
> cheers
>