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Comment by bradfa | Hacker News Reader

If you want devtmpfs mounted automatically by the kernel within an initramfs, then you may need to patch your kernel slightly, like: https://lore.kernel.org/lkml/25e7e777-19f9-6280-b456-6c9c782...

By default the kernel will not mount devtmpfs automatically within an initramfs, even if you've configured your kernel to automatically mount devtmpfs, because it waits until the real rootfs has been mounted to do it. This is fine unless you never plan to actually mount a real rootfs as you've made your initramfs have everything a normal rootfs would have and you want devtmpfs to Just Work.

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Thanks--you just saved me an hour or two of debugging. I thought, CONFIG_DEVTMPFS_MOUNT=y, what more do I need to do? I should've just RTFM though: even an unpatched kernel mentions this in the help for that config option.

It's also easy enough to just add a mount command to your initramfs init, or embed required device nodes in the CPIO.

It is, unless you're just trying to use an off the shelf distro as your initramfs and that distro's default init choice makes the assumption that the kernel will mount devtmpfs for it.

My understanding is that pretty much every modern init system has a service for mounting the devtmpfs, even if it's not in the fstab, and depending on the switch_root binary being used, mounts made in the initramfs may or may not be re-mounted in the new root.

You don't even need a devtmpfs in the initramfs environment, and it may or may not even persist into the fully booted system. I believe Systemd has a tmpfs unit file, and OpenRC has a service for it which is enabled by default.

You don't even need a devtmpfs in the initramfs environment, and it may or may not even persist into the fully booted system.

In this case, my point is for a system where the "fully booted" state is still within an initramfs. There is no "new root" past the initramfs, it IS the rootfs which will run forever until the system is rebooted or shutdown.

You can just mount then exec the real init, no?

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