Configuration.nix options for filesystem ignored despite being in fstab

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1

I import the following in my configuration.nix, but the gid/mode options have no effect, even after reboot, even though /etc/fstab reflects the changes!:

{ config, lib, pkgs, ... }:

{
  boot.kernel.sysctl = {
    "vm.nr_hugepages" = 512;  # Adjust the number based on your needs
  };

  fileSystems."/dev/hugepages" = {
    device = "hugetlbfs";
    fsType = "hugetlbfs";
    options = [ "mode=1770" "gid=100" ];  # 100 is typically the GID for the "users" group
  };
}

With /etc/fstab:

/dev/disk/by-uuid/323ccc87-771f-4b0b-bf39-c6e696d8db7e / btrfs x-initrd.mount,subvol=root,compress=zstd 0 0
/dev/disk/by-uuid/4C9A-B83E /boot vfat fmask=0077,dmask=0077 0 2
hugetlbfs /dev/hugepages hugetlbfs mode=1770,gid=100 0 2
/dev/disk/by-uuid/323ccc87-771f-4b0b-bf39-c6e696d8db7e /home btrfs subvol=home,compress=zstd 0 0
/dev/disk/by-uuid/323ccc87-771f-4b0b-bf39-c6e696d8db7e /nix btrfs x-initrd.mount,subvol=nix,compress=zstd,noatime 0 0

Yet:

$ ls -ld /dev/hugepages
drwxr-xr-x 2 root root 0 May  3 12:45 /dev/hugepages

There is a 4 year old thread where somebody is trying to do this for a different filesystem type, but it has no answers, which is surprising to me because this doesn’t seem like this should be that far off the beaten path.

Questions:

  • Why doesn’t NixOS respect its own changes to /etc/fstab?
  • How are you supposed to do this instead?
2

That’s showing you the permissions of the mount point. Usually, mode and gid parameters on filesystems affect the contents of the filesystem, not the mount point itself. Do the contents of /dev/hugepages have the correct bits?

1 Like
3

Oh, what’s the right way to change the permissions of the mount point itself? I want to do this:

$ touch /dev/hugepages/foo
touch: cannot touch '/dev/hugepages/foo': Permission denied

There’s no contents in there to start (it’s a memory only filesystem), so I have to switch to root to make something in there, and then the file that’s created is still user root group root. I could chown/chgrp obviously but I’m trying to set things up so root isn’t necessary.

4

Not sure; this particular mount is probably being managed by systemd, and I don’t know how it’s handling any conflicts between its built-in hugepages mount unit and /etc/fstab.

You might be able to customize it by creating a systemd.mounts entry with the exact same name ("dev-hugepages.mount").

Or you might need to experiment with suppressing the built-in mount by adding it to systemd.suppressedSystemUnits.

1 Like
5

This is incorrect. The permissions are a property of the file system, not the mountpoint it was mounted at. That file/directory that was there before the file system was mounted is now “beneath” the mountpoint and its permissions are irrelevant.

@jgarvin The issue is indeed because /dev/hugepages is an upstream systemd mount unit, which overrules fstab. The reason it’s overruled is because NixOS puts those upstream units in /etc while systemd-fstab-generator puts its fstab-derived units in /run, and /etc overrules /run in systemd logic. Other distros put these upstream mount units in /usr/lib, which /run overrules, so they get the opposite behavior.

But yea, point is, you need to override it in the mount unit. Something like this should work:

systemd.mounts = [
  {
    what = "hugetables";
    where = "/dev/hugepages";
    options = "mode=1770,gid=100";
  }
];
3 Likes
6

thanks, that did the trick!