I think it could be solved outside the OS, but the challenge is that you would need to define some common APIs and get them into the standard libraries of progr

I think it could be solved outside the OS, but the challenge is that you would need to define some common APIs and get them into the standard libraries of programming languages. You would need a service API so that you can plug in the tagging service backend of your choice (or something that comes with the OS). There is nothing wrong with having a userland tag management service. In fact, you'd probably want it in userspace if possible. Implementing a proof of concept for this would have been easy if it wasn't for the fact that getting dirent to inode is fast and getting from inode to dirent(s) is very much not fast (since there is a risk that the file may be renamed).