Is udev also a pseudo-filesystem type?

On an embedded device running GNU/Linux with a custom BSP, when I type

df /dev

in Filesystem column I get

devtmpfs
ps -A | grep udev 

gives

224 ?      00:00:00 systemd-udevd

In a PC running Ubuntu 22.04, the same commands give

udev

and

744 ?      00:00:02 systemd-udevd

respectively.

My understanding is that the embedded device is using a combination of devtmpfs creating device nodes + systemd-udevd managing ownership and permissions on top of it. Therefore devtmpfs as the filesystem type of /dev.
On the other hand, Ubuntu is running udev with a dedicated udev pseudo-filesystem type.

  1. Is this correct?
  2. Is udev (besides being the whole solution with a daemon running in userspace) also a specific pseudofilesystem type?
  3. I thought that "classic" udev configuration used a generic tmpfs for /dev. Is this wrong?
Asked By: mrn

||
  1. No (see below).

  2. df shows the mount device; pseudo-file systems don’t have one, and their name is arbitrary. If you run mount you’ll see the actual file system type, which is devtmpfs on Ubuntu as well.

  3. udev used to use a generic tmpfs, but since devtmpfs’s creation in 2009, /dev uses the latter.

Answered By: Stephen Kitt
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