after creating a snapshot, subvol can't be remounted ro, but unmounted

[Christoph Anton Mitterer <calestyo@scientia.org>]

Hey.

I've experience this for ages and was always too lazy to report it (and
it's not really big problem anyway):

That's all one btrfs, with a subvol at <fsroot>/data/ which is mounted
a / and the <fsroot>/ being a noauto,ro mountpoint at /data/system/.


root@heisenberg:~# mount /data/system/
root@heisenberg:~# cd /data/system/
root@heisenberg:/data/system# btrfs subvolume snapshot -r data/ snapshots/data/2026-08-21_1_live/
ERROR: Could not create subvolume: Read-only file system
root@heisenberg:/data/system# mount -o remount,rw /data/system/
root@heisenberg:/data/system# btrfs subvolume snapshot -r data/ snapshots/data/2026-08-21_1_live/
Create readonly snapshot of 'data/' in 'snapshots/data/2026-08-21_1_live/'

After I created a snapshot, I cannot remount,ro it anymore (even if I
wait longer and try multiple times:

root@heisenberg:/data/system# mount -o remount,ro /data/system/
mount: /data/system: mount point is busy.
       dmesg(1) may have more information after failed mount system call.

even if I cd somewhere else:

root@heisenberg:/data/system# cd
root@heisenberg:~# mount -o remount,ro /data/system/
mount: /data/system: mount point is busy.
       dmesg(1) may have more information after failed mount system call.

But what immediately works (even wit cwd /data/system/) is unmounting:

root@heisenberg:~# umount /data/system 
root@heisenberg:~# 


No idea whether this might be caused by any higher-level stuff (udisks
or so?) but my intuitive guess was that it might actually be something
in the fs itself.

Thanks,
Chris.