I have an upcoming trip to Shanghai where I'll be staying at a hotel. In the middle of the trip, I need to go to Hanoi for 2 days, and then return. In most countries, I would book the hotel room for the extra two days, so I can keep my personal belongings there and just return to the same room. But I know that in China one has to register stays with the police, which the hotel does. If I keep the room, and don't check-out, I can imagine the hotel won't re-register my address when I return from Hanoi 2 days later.

Does anyone know if such a plan is even possible to do in China? Or is it just better for me to check-out and re-check-in? Or could I just do the registration myself?

EDIT: Just to add, I know that Shanghai has a website for registering stays.