The important point to understand is that the page structure is associated with physical pages,not virtual pages.
If a page is not free, the kernel needs to know who owns the page. Possible owners include user-space
processes, dynamically allocated kernel data, static kernel code, the page cache, and so on.
Zones
the kernel divides pages into different zones.
ZONE_DMA
ZONE_DMA32
ZONE_NORMAL
ZONE_HIGHMEM
Note that the zones do not have any physical relevance but are simply logical
groupings used by the kernel to keep track of pages.
