Even I did it many times ,but I have to write down the notes in case I forget it!
1. boot up Linux os with rescue mode .
2. make sure which partition / resides and type the command
cryptsetup luksOpen /dev/sda2 devicename
3. varyon system vg
vgchage -ay
4. Please make sure lvsize > filesytem size ,then type the command
lvresise -L lvname -r
</code>
Git it done now :)
Redhat Disk Encrption