In a fresh install of 15.10 server, kernel 4.2.0-16-generic, I'm seeing the following entries in my kern.log file:
Feb 1 11:49:30 Client-5 kernel: [ 117.847487] MCE 0x1182: reserved kernel page still referenced by 1 users
Feb 1 11:49:30 Client-5 kernel: [ 117.847501] MCE 0x1182: recovery action for reserved kernel page: Failed
What does this mean? Is it important? How can one find more information?
From a quick read of the source code, it appears you've suffered an ECC-detected (but not correctable) two-bit corruption. And that corruption has hit some page in use by the kernel; other messages may give some clue as to what use.
It is important. Single-bit (correctable) errors happen sometimes, especially with high-density memory. Double-bit (uncorrectable) shouldn't happen—probably you need to find the failing memory module and replace it.
Since some bit of kernel memory is corrupt, if you can't immediately pull the machine from service for hardware troubleshooting, you should probably schedule an emergency reboot.
