free_area_init()

This is where the global `mem_map` array is set up and zone information is added and the `pgdat` structure. It is important that we understand the zoned buddy allocator before we proceed. So please read the zoned buddy allocator (in the next section) introduction and come back here.

The mem_map array is set up and the zone list is built here. The algorithm followed is given here.

• Calculate the total number of pages. i.e, pages in all the zones.
• Calculate the holes present in memory.
• Calculate the actual number of pages using the above two values (i.e, calculate the difference).
• Check weather mem_map is already allocated. If not allocate necessary memory (number of pages times the size of 'struct page') from the bootmem handler.
• Set up other necessary fields in pgdat.
• Reserve all the memory by using the set_page_count and SetPageReserved calls
• For each zone present,
  • Set up the necessary fields in the zone_t structure in pgdat.
  • Set up the virtual address for all the pages to the initial value. Once allocation starts this will start changing as new allocations are made.
  • Set up the buddy system bitmaps in the zone.
• Build the allocation fall back lists using the build_zonelist call.