Yep, this two layer approach is also desirable since in userspace you aren't context switching to the kernel all the time and can make some assumptions to avoid

Yep, this two layer approach is also desirable since in userspace you aren't context switching to the kernel all the time and can make some assumptions to avoid certain locking and external fragmentation with the kmem. Short lived allocs are really cheap coming out of i.e. jemalloc arenas and that is important for a lot of programming ergonomics.