Comment # 26 on bug 1156421 from Fabian Vogt

(In reply to Goldwyn Rodrigues from comment #25)
> We had a discussion in the btrfs mailing list and it was concluded that
> these changes would deviate from the "default" case [1]. Currently sys
> admins are setting any subvolume read-only to set the root filesystem
> read-only. While this is not ideal, this behavior has already been exploited.
> 
> [1]
> https://lore.kernel.org/linux-btrfs/20220211164422.GA12643@twin.jikos.cz/T/#t

Ah, good ol' spacebar heating (https://xkcd.com/1172/).

Maybe the example doesn't really show the severity of the issue, as it focuses
on the "root" mountpoint. This has more independent mounts of subvolumes:

dd if=/dev/zero of=btrfsfs seek=240 count=0 bs=1M
mkfs.btrfs btrfsfs
mkdir mnt
mount btrfsfs mnt
btrfs subvol create mnt/sv0
btrfs subvol create mnt/sv1
umount mnt

mkdir sv{0,1}mnt
mount -o subvol=/sv0 btrfsfs sv0mnt
mount -o subvol=/sv1 btrfsfs sv1mnt
findmnt sv0mnt # RW
findmnt sv1mnt # RW
mount -o remount,ro sv0mnt
findmnt sv0mnt # RO
findmnt sv1mnt # RO!
mount -o remount,rw sv1mnt
findmnt sv0mnt # RW!
findmnt sv1mnt # RW
umount sv*mnt

Do we have any filesystems we can compare the behaviour with?