Most Chinese metropolises don't make it "welcome" to migrants from outside administrative region in the fist place.

That said, it's a hard problem to tackle. You can't have 25 million people swarm your city and be able to manage things properly to ensure safety and proper services, but additionally, it's hard to economically engineer even economic development --which is the root cause of the issue. Migrants come because their prospects in the big megalopolises are better even while it being entailing administrative disadvantages.