Many kernels does memory management fully in kernel space, others separate it and offer non-trivial API for doing this in userspace.
We're have many restrictions - security and performance. If you will targeting to one of this properties you will loose on other. If you want to have a very strong and secure mechanism you will make a many context switching within kernel and user spaces - it will slow down your system perfomance, if you will don't care about security you will make it faster ...
I decide to make a basics virtual memory manager operations within kernel space, and offer simple API (just 4 calls) - address_space_get(), address_space_set(), address_space_alloc(), address_space_release(). And ... bind page fault exception to userspace area virtual mm server.
In this scheme microkernel just mark/unmark address space, it doesn't makes a check (and it will works fast) , and microkernel doesn't care about page fault exception - it's a server headache.
The security implemented on userspace - with ACL server that cannot be exchanged - like other varios servers (you must load its via multiboot modules way), all requests to ACL cached to the IPC cache server (it cannot be exchanged too) to avoid premiary access violation checks and grow up speed of requests. On the figure below I've tried to show it graphically (I like this, but I don't like to make a graphics ...):
User task just trying to take some virtual memory and extend it's address space - it's a deal of libs to make all job with calling of vmm server. Vmm server asks about access rights/limits/etc ... on ACL server, if all granted - vmm will allocate space for user task, if will be need - it will calls MuiString microkernel via simple API.
This structure allows to make secure and fast (compairing with some highly secured or highly fast microkernel systems) - it's a good design for implementing different models of memory allocations (real-time, preemption, so on ...)