vfs: add one workqueue to update hot map info

  Add a workqueue per superblock and a delayed_work
to run periodic work to update map info on each superblock.
  Two arrays of map list are defined, one is for hot inode
items, and the other is for hot extent items.
  The hot items in the RB-tree will be at first distilled
into one temperature in the range [0, 255]. If it is old,
it will be not linked or aged out, otherwise then it will
be linked to its corresponding array of map list which use
the temperature as its index.

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

  Register a shrinker to control the amount of memory that
is used in tracking hot regions. If we are throwing inodes
out of memory due to memory pressure, we most definitely are
going to need to reduce the amount of memory the tracking
code is using, even if it means losing useful information
That is, the shrinker accelerates the aging process.

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

  Add a directory '<dev_name>' in /sys/kernel/debug/hot_track/
for each volume that contains four files which are 'inode_stat',
'extent_stat', 'inode_spot', and 'extent_spot'.

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

  FS_IOC_GET_HEAT_INFO: return a struct containing the various
metrics collected in hot_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.

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

  Add two proc interfaces hot-age-interval and hot-update-interval
under the dir /proc/sys/fs/ in order to turn HOT_AGE_INTERVAL and
HOT_UPDATE_INTERVAL into be tunable.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
btrfs: add hot tracking support

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

Reviewed-by:   David Sterba <dsterba@suse.cz>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add documentation

  Add Documentation for VFS hot tracking feature

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

  Introduce one ability to enable that specific FS
can register its own hot tracking functions.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add one list_head field

  Add one list_head field 'reloc_list' to accommodate
hot relocation support.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
btrfs: add one new block group

  Introduce one new block group BTRFS_BLOCK_GROUP_DATA_SSD,
which is used to differentiate if the block space is reserved
and allocated from one HDD disk or SSD disk.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
btrfs: add one task for hot relocation

   Add one private task for hot relocation. It will check
if there're some extents which is hotter than the threshold
and queue them at first, if no, it will return and wait for
its next turn; otherwise, it will check if SSD ratio is beyond
beyond its usage threshold, if no, it will directly relocate
those hot extents from HDD disk to SSD disk; otherwise it will
find the extents with low temperature and queue them, then
relocate those extents with low temperature and queue them,
and finally relocate the hot extents from from HDD disk to SSD
disk.

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

  Add three proc interfaces hot-reloc-interval, hot-reloc-threshold,
and hot-reloc-max-items under the dir /proc/sys/fs/ in order to
turn HOT_RELOC_INTERVAL, HOT_RELOC_THRESHOLD, and HOT_RELOC_MAX_ITEMS
into be tunable.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
btrfs: add hot relocation support

  Add one new mount option '-o hot_move' for hot
relocation support. When hot relocation is enabled,
hot tracking will be enabled automatically.
  Its usage looks like:
    mount -o hot_move
    mount -o nouser,hot_move
    mount -o nouser,hot_move,loop
    mount -o hot_move,nouser

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