Document I_SYNC and I_DATASYNC

After some archeology (see http://logfs.org/logfs/inode_state_bits) I
finally figured out what the three I_DIRTY bits do.  Maybe others would
prefer less effort to reach this insight.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/fs.h

@@ -1279,8 +1279,10 @@ struct super_operations {
  *
  * Two bits are used for locking and completion notification, I_LOCK and I_SYNC.
  *
- * I_DIRTY_SYNC		Inode itself is dirty.
- * I_DIRTY_DATASYNC	Data-related inode changes pending
+ * I_DIRTY_SYNC		Inode is dirty, but doesn't have to be written on
+ *			fdatasync().  i_atime is the usual cause.
+ * I_DIRTY_DATASYNC	Inode is dirty and must be written on fdatasync(), f.e.
+ *			because i_size changed.
  * I_DIRTY_PAGES	Inode has dirty pages.  Inode itself may be clean.
  * I_NEW		get_new_inode() sets i_state to I_LOCK|I_NEW.  Both
  *			are cleared by unlock_new_inode(), called from iget().
@@ -1312,8 +1314,6 @@ struct super_operations {
  *			purpose reduces latency and prevents some filesystem-
  *			specific deadlocks.
  *
- * Q: Why does I_DIRTY_DATASYNC exist?  It appears as if it could be replaced
- *    by (I_DIRTY_SYNC|I_DIRTY_PAGES).
  * Q: What is the difference between I_WILL_FREE and I_FREEING?
  * Q: igrab() only checks on (I_FREEING|I_WILL_FREE).  Should it also check on
  *    I_CLEAR?  If not, why?