Cannot rename root dataset despite "zfs set mountpoint=/"

[ChristophSchmidpeter]

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System information

Type	Version/Name
Distribution Name	Arch Linux
Distribution Version	latest
Kernel Version	5.19.7
Architecture	x64
OpenZFS Version	2.1.5

Describe the problem you're observing

I boot Linux from a ZFS root filesystem. However, despite having the mount point set explicitly, I cannot rename the dataset from within.

Let's assume my root dataset is MyPool/MyRootFs. Before booting into it, I set the mountpoint as mentioned explicitly (via zfs set mountpoint=/ MyPool/MyRootFs), and then I boot into it. Thus, zfs get MyPool/MyRootFs returns:

zfs get MyPool/MyRootFs

NAME                PROPERTY      VALUE     SOURCE
MyPool/MyRootFs     mounted       yes       -
MyPool/MyRootFs     mountpoint    /         local

Trying to rename MyRootFs fails:

zfs rename MyPool/MyRootFs MyPool/MyRootDataset
cannot unmount '/': pool or dataset is busy

Given mount point is pegged to '/', and thus the mount point does not change (i.e. file "/path/to/my/file.txt" before the rename should still point to "/path/to/my/file.txt" after the rename). As a result, the filesystem being "busy" should not be an issue here, as only the dataset name changes from a ZFS perspective, not, however, the mount point from a OS / root FS perspective. Therefore, I believe the rename operation above should succeed.

Describe how to reproduce the problem

See above.

Include any warning/errors/backtraces from the system logs

(100% reproducible, but if needed I can generate logs once I know which logs help here)

[IvanVolosyuk]

You can try 'zfs rename -u'. But I agree that if explicit mountpoint property specified renaming shouldn't change any mounts and should be allowed.

[ChristophSchmidpeter]

@IvanVolosyuk Thanks for the hint. That's a good workaround for now.

Your comment brought me to an idea. I think the zfs rename's error message

cannot unmount '/': pool or dataset is busy

might be more useful if it suggested to use the -u parameter

[GregorKopka]

I think the zfs rename's error message might be more useful if it suggested to use the -u parameter

I think that zfs rename should not try to remount in case the effective mountpoint does not change.

[akorn]

Note: zfs set mountpoint= has the same problem. You can't use it to explicitly set the mountpoint on a busy filesystem to what its current inherited value is unless you specify -u.

I also think that if the effective mountpoint doesn't change, the umount shouldn't be attempted in the first place; but that at the very least, the error message should point the user towards -u.