To Do's

1. Signal handling in a kernel thread
2.

Locks while extracting pages from Free List (PCP and Buddy)

access pcp cache
----------------
get_cpu()
local_irq_save()
zone lock (nex)
access buddy to replenish
zone unlock (nex)
local_irq_restore()
put_cpu()

access buddy
------------
get_cpu()
zone lock (ex)
zone unlock (ex)
put_cpu()

zone_lock_irqsave: local_irq_save + zone_lock
zone_unlock_irqrestore: zone_unlock + local_irq_restore

* when a page is given to pcp, the page private is set to migration type.