Hi btrfs maintainers, I'd like to discuss about the possibility of having btrfs allowing to mount any of a set of different devices holding the same filesystem, i.e, a duplicate fsid scenario. For example, imagine sda1 and sdb1 both contains the same filesystem, I'd like to be able to mount either sda1 OR sdb1 (not both at the same time!); currently, only the first that was created is able to get mounted. First of all, let me describe the use case I have in mind: the Steam Deck gaming console makes use of an A/B double side partitioning scheme and a RO btrfs rootfs is one of the partitions (in each "side"). So, we could have a scenario of some user that wishes to have the same image in both "sides", perhaps to test some modifications for example and have the pristine/unmodified image in the other one. Before using btrfs, Steam Deck was using ext4 and everything worked fine in such scenario, but with btrfs it's impossible to (even explicitly) mount the newer partition with the same fsid. We also have the use case of images' upgrading process, that in a recovery configuration might end-up installing the same (pristine) image in the other "side"; that case is kinda trivial one that we'd expect to work. Notice that a reason behind this duplication in the images' fsid is because they're signed rootfs snapshots, so wouldn't so trivial/desirable to de-duplicate them in some cases. By checking the code, we can see that the validations preventing such mounting operation happens in device_list_add(): when udevd checks the filesystems on boot time, the older partition (the first one with the given fsid) is verified and the btrfs_fs_devices struct is null - hence it is created along with the corresponding btrfs_device for such partition/device, which is added in the fs_devices list. When the newer partition gets scanned, fs_devices exists, so the function btrfs_find_device() is invoked and finds the first btrfs_device, which differs from the one we are trying to mount, effectively leading to this second device being impossible to mount. Since I'm not an expert in the btrfs inner workings, I'd like some advice to understand if some mount flag would be accepted to enable this behavior; maybe I'm not seeing some subtle issue, but given the 2-side nature of our setup, the explicit mount point control, and the fact we don't expect to use any btrfs more "complex"features that might be confused by non-unique fsids (like RAID), seems feasible to allow such btrfs behavior. Notice we don't require to change the default, hence the idea to have it as a mount flag or something like that - suggestions are greatly appreciated. Thanks in advance, Guilherme
