Right, it's all very clear now. At some point in the future, I'd like 
to visit why SPARSEMEM-based hot-add is not always used but it's a 
separate issue.

quoted

The add areas
are marked as RESERVED during boot and then later onlined during add.

That explains the reserve_bootmem_node()

But pages are marked reserved by default. You still have to alloc the 
bootmem map for the the whole node range, including reserve hot add 
areas and areas beyond e820-end-of-ram. So all the areas are already 
reserved, until freed.

--Mika