Forked GC explained

[frame]

I'm not sure I fully understand how it works. I know that the OS 
creates read only memory pages for both and if a memory section 
is about to be written, the OS will issue a copy of the pages so 
any write operation will be done in it's own copy and cannot mess 
up things.

But then is the question, how can memory be marked as free? The 
forked process cannot since it writes into a copy - how it is 
synchronized then?

Is the GC address root somehow shared between the processes? Or 
does the forked process communicate the memory addresses back to 
the parent?

If so, does the GC just rely on this?

Are freeing GC operations just locked while the forked process is 
running?

What happens if a manually `GC.free()` is called while the forked 
process marks the memory as free too but the GC immediately uses 
the memory again and then gets the notification to free it from 
the forked child? Can this happen?