On systems that boot without initrd on a btrfs root file systems the BTRFS_IOC_DEV_INFO ioctl returns /dev/root as backing device. That sucks, since that is not a real device visible to userspace. Since this has been that way since forever, and it doesn't look like the kernel will get fixed soon for this, let's at least generate a useful error message in this case. This is not a bug fix, just a tweak to make this more recognizable. Once the kernel gets fixed to report the correct device nodes in this case, in a way userspace can make sense of them things will magically work for systemd, too. (Note that this doesn't add a log message about this to really all cases we call get_device() in, but just the main ones that are called in early boot context, after all all there's no benefit in seeing this message too many times.) https://github.com/systemd/systemd/issues/16953 https://bugs.freedesktop.org/show_bug.cgi?id=84689 https://bugzilla.kernel.org/show_bug.cgi?id=89721
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