Page Tables

the page->ptl is a spinlock which is used when this page is being used as a pmd full of ptes. this pmd page can be in high memory or normal zone.

there are two reserved fixmap vaddrs (KM_PTE0,1) to map the pmd page if you want the pmd page be in high memory addr.

pte_offset_map_lock gets you the pmd page table (locked) and the lock itself.
if we want to change any pte inside this table, we must have the lock on it. after finishing work, unlock the key. (use pte_unmap_unlock)