Is user mode linux (UML) project stopped?

While it does look stale, it still works as of v4.9.6. Tested with this setup: https://unix.stackexchange.com/questions/73203/how-to-create-rootfs-for-user-mode-linux-on-fedora-18/372207#372207

If you do:

git log arch/um

it shows a few hits per month, mostly maintenance to keep up with the rest of the kernel.

I believe that the reason for the slowdown is due to the rise of other better virtualization methods, specially:

• KVM, which QEMU can use with -enable-kvm, useful for full system simulation
• cgroups and namespaces, which used by Docker, useful for lightweight containers

Both of these methods execute most CPU instructions directly on the host, and are therefore very fast.

Those two methods are very likely better options for most applications than UML nowadays.

See also: https://unix.stackexchange.com/questions/127829/why-would-someone-want-to-run-usermode-linux-uml

UML support is now part of mainstream Linux kernel. Search string UML from here: https://github.com/torvalds/linux