Re: [PATCH 1/3] btrfs: derive f_fsid from on-disk fsuuid and dev_t

[David Sterba <dsterba@suse.cz>]

On Sat, Sep 12, 2026 at 06:19:21PM +0800, Anand Suveer Jain wrote:
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1836,8 +1836,12 @@ static int btrfs_statfs(struct dentry *dentry,
> struct kstatfs *buf)
>         f_fsid.val[0] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root)
> >> 32;
>         f_fsid.val[1] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root);
> 
> -       /* Hash dev_t to avoid f_fsid collision with cloned filesystems. */
> -       if (fs_info->fs_devices->total_devices == 1) {
> +       /*
> +        * Hash dev_t to avoid f_fsid collisions with cloned filesystems.
> +        * Only do this when a clone is present so the original filesystem
> +        * (mounted first) maintains backward-compatible f_fsid behavior.
> +        */
> +       if (fs_info->fs_devices->temp_fsid) {
>                 __kernel_fsid_t dev_fsid =

This should fix the most common case where people don't have any cloned
filesystems at all, so I guess this is good as a fix. Please send it so
I can add it to this -rc pull request. We can try to catch next week's
release of stable trees.

Related to the cloned filesystems, it can get complicated if the
original filesystem gets unmounted and mounted again, now it would have
the temp_fsid set, right?

I don't see the temp_fsid documented (btrfs-progs) other than in the
changelogs and briefly in the sysfs file. This should be more visible as
a separate feature and with the known use cases.