Re: [PATCH] btrfs: derive f_fsid with dev_t only when temp_fsid is active

[David Sterba <dsterba@suse.cz>]

On Mon, Sep 14, 2026 at 04:28:25PM +0800, Anand Suveer Jain wrote:
> 
> 
> >> . For stable kernels, this flag will keep the new f_fsid derivation
> >> disabled by default.
> 
> 
> > No. That still gives you a broken fsid on upgrading to the next major kernel, doesn't it?
> 
> Upgrading to a _major_ kernel would require an extra step to re-encrypt
> with the new FSID _for some configs_.
> A one-time operational overhead.

It's fairly common to boot different kernel versions on the same system,
so the upgrade is perhaps installing it but the assumptions about what's
working do not change. I have mixed results upgrading kernel and Nvidia
drivers and imagining that I'd have to re-encrypt anything just between
reboots sounds like usability antipattern.

> > Maybe a new flag on the file system itself to indicate that its
> > stable FSID is computed using the new scheme.
> 
> A mkfs-time or runtime switch, something like:
>   /sys/fs/btrfs/<UUID>/enable_collision_free_fsid
> (just as an example) is an option.

Even as a conceptual idea this does not improve things, users do not
care about this level of detail. We already have enough problems
defining correct behaviour with fsid, metadata uuid in combination with
the temp_fsid and seeding.