When pmap_enter_l2() needs to destroy an existing kernel superpage mapping, do not remove the saved page table page (PTP) from the pm_root trie and remap it into the page table. Instead, simply zero it. Then, later the PTP does not need to be unmapped and reinserted into the trie. If the kernel region is not mapped by a superpage, then try to insert the PTP into the pm_root trie before clearing the PTEs. If the PTP insertion fails, then we can return failure with the old mappings still in place. Convert an assertion from a panic to a KASSERT. Reviewed by: kib, markj (an earlier version) MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D51220