Some optimization in current kernel

1. for do_mpage_readpage, we can introduce a flag in page->flag,

indicating whether we have a continous block in the disk,

then the next time we can get the actual sector much faster based

on previous sector, in this way we don't have to leverage the driver

to search for actual sector.

http://blog.csdn.net/chenyu105/article/details/8604149

2. for scan_swap_map, each time we try to find a free page slot,

we check by SWAPFILE_CLUSTER pages, but I think we can check

the free page slot by bitmask.

4. we can write a tool to enter different cstate in round-robin manners.