vfs: add parsing support for one new mount option

  Introduce one new mount option '-o hottrack',
and add its parsing support.
  Its usage looks like:
   mount -o hottrack
   mount -o nouser,hottrack
   mount -o nouser,hottrack,loop
   mount -o hottrack,nouser

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add init and exit support

  Add initialization function to create some
key data structures when hot tracking is enabled;
Clean up them when hot tracking is disabled.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: introduce hash head structure

  Adds a hash table structure to efficiently lookup
the data temperature of a file or its range.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: introduce hash node structure

  Define one hash list node, it will keep track of
temperature info and be stored in each hash list
of hash table.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: initialize kmem cache for hash node

  Initialize kmem cache for hot_hash_node
and its initialization utils helpers.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: initialize rwlock for each hash list

  Initialize rwlock for each hash list of hash table.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: enable hot data tracking

  Miscellaneous features that implement hot data tracking
and generally make the hot data functions a bit more friendly.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: fork one kthread to update temperature info

  Fork and run one kernel kthread to calculate
that temperature based on some metrics kept
in custom frequency data structs, and store
the info in the hash table.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add support for 3 new ioctl interfaces

  FS_IOC_GET_HEAT_INFO: return a struct containing the various
metrics collected in btrfs_freq_data structs, and also return a
calculated data temperature based on those metrics. Optionally, retrieve
the temperature from the hot data hash list instead of recalculating it.

  FS_IOC_GET_HEAT_OPTS: return an integer representing the current
state of hot data tracking and migration:

0 = do nothing
1 = track frequency of access

FS_IOC_SET_HEAT_OPTS: change the state of hot data tracking and
migration, as described above.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add debugfs support

  Add a /sys/kernel/debug/hot_track/<device_name>/ directory for each
volume that contains two files. The first, `inode_data', contains the
heat information for inodes that have been brought into the hot data map
structures. The second, `range_data', contains similar information for
subfile ranges.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: testcase, direct io read

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: testcase, direct io write

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: testcase, direct io read/write

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: testcase, cached io read

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: testcase, cached io write

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: testcase, new ioctl interface

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>