vfs: introduce private radix tree structures

  One root structure hot_info is defined, is hooked
up in super_block, and will be used to hold rb trees
root, hash list root and some other information, etc.
  Adds hot_inode_tree struct to keep track of
frequently accessed files, and be keyed by {inode, offset}.
Trees contain hot_inode_items representing those files
and ranges.
  Having these trees means that vfs can quickly determine the
temperature of some data by doing some calculations on the
hot_freq_data struct that hangs off of the tree item.
  Define two items hot_inode_item and hot_range_item,
one of them represents one tracked file
to keep track of its access frequency and the tree of
ranges in this file, while the latter represents
a file range of one inode.
  Each of the two structures contains a hot_freq_data
struct with its frequency of access metrics (number of
{reads, writes}, last {read,write} time, frequency of
{reads,writes}).
  Also, each hot_inode_item contains one hot_range_tree
struct which is keyed by {inode, offset, length}
and used to keep track of all the ranges in this file.

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: add function to update access frequency

  Add some utils helpers to update access frequencies
for one file or its range.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: introduce one map array table

  Adds a array table structure which contains
a lot of list and is used to efficiently
look up the data temperature of a file or its
ranges.
  In each list of array table, the array node
will keep track of temperature info.

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: add function to update the temperature map

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add aging function for out-of-date temperature info

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

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add 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 memory shrink function

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: add documentation

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