Not a Btrfs subvolume. It's a Bind Mount. nixos-generate-config.pl gets it wrong.

[Sepero]

Describe the bug

On a Btrfs filesystem, the perl code below does not understand the difference between a mounted subvolume, and a regular directory that has been bind mounted.

nixpkgs/nixos/modules/installer/tools/nixos-generate-config.pl

Line 440 in f199d57

die "Failed to retrieve subvolume info for $mountPoint\n";

Steps To Reproduce

Having a bind mounted /nix or /boot directory causes rebuild to fail.

Expected behavior

Recognize that directory is bind mounted, and not a subvolume.

Related

Possibly related bugs. #53177 #341481

Notify maintainers

@bjornfor @anthonyroussel @Atemu @Mic92 @lheckemann @roberth @mberndt123 @ajs124 @name-snrl @lukegb @RaitoBezarius @SuperSandro2000 @Luflosi @catap @Ma27 @Artturin @worldofpeace @edolstra @prusnak

[bjornfor]

Having a bind mounted /nix or /boot directory causes rebuild to fail.

Do you manually bind mount /nix or /boot in the installer? If so, why?

(When I started using NixOS there was this idea that nixos-generate-config should/could be re-run to update after hardware changes, but IME that never worked; it generated fileSystems config that I didn't want in hardware-configuration.nix. So I never run it outside of the installer environment. And in the installer environment the mounts are much simpler / well known.)

[ElvishJerricco]

Do you manually bind mount /nix or /boot in the installer? If so, why?

The main issue here isn't so much to do with the specific mounts, but rather that any bind mount you'd like to configure cannot come from a btrfs source, or else nixos-generate-config is going to get something wrong. It'll either yell that it isn't a (sub)volume, or it'll create a bad bind mount if it's actually incidentally a bind mount.

TL;DR: We're not really handling bind mounts right here at all.

[Sepero]

The script nixos-generate-config.pl does not appear to intelligently acknowledge the existence of bind mounted directories on Btrfs.