This is just a quick explaination of the problems fixed by the page table
races patch.

1. use kmap_atomic in clear_user_*page
	- fixes potential schedule while holding page table spinlock
	under heavy load.
2. double spin_unlock fix in memory.c
	- code could spin_unlock twice under rare alignments while copying
	page tables
3. make map_user_kiobuf use read lock
	- prevents stalls during page faults waiting for io to complete
4. move page copy/clear out from page table spinlock
	- reduce spinlock contention