There are large space in f2fs inode, so last inline data patch set have made
f2fs to acquire the ability of storing data of small file (less than ~3.4k)
directly in inode block, but not support inline dir. In this patch, we make
f2fs to support inline dir.

Inline dir layout is the same as inline data layout,
a. i_addr[0] are reserved space for converting from inline dir to regular one
when out-of-space occur in inline dentry.
b. i_addr[1..872] will be used as storing space of inline dentry.
c. i_addr[873..922] are reserved for inline xattr.

Since our max size of inline dir space is limit to MAX_INLINE_DATA (3488 
we introduce inline dentry struct fit for available space, then our dentry slot
number in inline dentry reduce to 182, less than normal dentry block's 214.

In inline dir mode, our dir entries will be lookuped/stored/deleted in the
inline dentry space of inode, util there are no more space to store new added
dir entry, in this case we will convert inline dentry to normal 0-index dentry
block and disable inline dir mode for this inode.

Benefit points:
a) space saving
   1) Test with fsstress with special arguments, it can save about 7% space.
   time fsstress -c -p 20 -n 500 -l 10 -d /mnt/f2fs -w -f chown=0 -f creat=10
   -f dwrite=0 -f fdatasync=0 -f fsync=0 -f link=10 -f mkdir=10 -f mknod=10
   -f rename=0 -f rmdir=0 -f symlink=10 -f truncate=0 -f unlink=0 -f write=10 -S
                Dev             1K-blocks  Used    Available Use% Mounted on
   empty        /dev/sdb        20969472  1134600  19691512   6% /mnt/f2fs
   based        /dev/sdb        20969472  2331676  18494436  12% /mnt/f2fs
   patched      /dev/sdb        20969472  2247480  18578632  11% /mnt/f2fs
   2) Test with storing kernel src, it can save less than 1.4% space.
   empty        /dev/sdb        20969472  1134600  19691512   6% /mnt/f2fs
   based        /dev/sdb        20969472  1934656  18891456  10% /mnt/f2fs
   patched      /dev/sdb        20969472  1923400  18902712  10% /mnt/f2fs

b) performance
   Test with fsstress shows cost time reduce about 5%.
   time fsstress -d /mnt/f2fs -l 5 -n 1000 -p 20 -c -r
   based        135.652 s
   patched      129.256 s  (mount -o inline_dentry)

 o introduce f2fs_drop_nlink from f2fs_delete_entry to reuse code in
 o remove kmap/kunmap for inode page pointed out by Jaegeuk Kim.
 o introduce a new mount option inline_dentry for inline dir pointed out by
   Jaegeuk Kim.
 o rebase to last dev of f2fs (20140924).
 o retest performance/space-gain for inline dir excluding influence of inline

Chao Yu (6):
  f2fs: add infra struct and helper for inline dir
  f2fs: add a new mount option for inline dir
  f2fs: export dir operations for inline dir
  f2fs: add key function to handle inline dir
  f2fs: enable inline dir handling
  f2fs: update f2fs documentation for inline dir support

 Documentation/filesystems/f2fs.txt |   4 +
 fs/f2fs/dir.c                      | 103 +++++++----
 fs/f2fs/f2fs.h                     |  40 ++++-
 fs/f2fs/file.c                     |   2 +-
 fs/f2fs/inline.c                   | 347 +++++++++++++++++++++++++++++++++++++
 fs/f2fs/namei.c                    |  38 ++--
 fs/f2fs/recovery.c                 |   5 +-
 fs/f2fs/super.c                    |   7 +
 include/linux/f2fs_fs.h            |  19 ++
 9 files changed, 512 insertions(+), 53 deletions(-)


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