fs: correctly document the inode dirty flags

The documentation for I_DIRTY_SYNC and I_DIRTY_DATASYNC is a bit
misleading, and I_DIRTY_TIME isn't documented at all.  Fix this.

Link: https://lore.kernel.org/r/20210112190253.64307-3-ebiggers@kernel.orgReviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NJan Kara <jack@suse.cz>

include/linux/fs.h

@@ -2084,8 +2084,8 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
 /*
  * Inode state bits.  Protected by inode->i_lock
  *
- * Three bits determine the dirty state of the inode, I_DIRTY_SYNC,
- * I_DIRTY_DATASYNC and I_DIRTY_PAGES.
+ * Four bits determine the dirty state of the inode: I_DIRTY_SYNC,
+ * I_DIRTY_DATASYNC, I_DIRTY_PAGES, and I_DIRTY_TIME.
  *
  * Four bits define the lifetime of an inode.  Initially, inodes are I_NEW,
  * until that flag is cleared.  I_WILL_FREE, I_FREEING and I_CLEAR are set at
@@ -2094,12 +2094,20 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
  * Two bits are used for locking and completion notification, I_NEW and I_SYNC.
  *
  * I_DIRTY_SYNC		Inode is dirty, but doesn't have to be written on
- *			fdatasync().  i_atime is the usual cause.
- * I_DIRTY_DATASYNC	Data-related inode changes pending. We keep track of
+ *			fdatasync() (unless I_DIRTY_DATASYNC is also set).
+ *			Timestamp updates are the usual cause.
+ * I_DIRTY_DATASYNC	Data-related inode changes pending.  We keep track of
  *			these changes separately from I_DIRTY_SYNC so that we
  *			don't have to write inode on fdatasync() when only
- *			mtime has changed in it.
+ *			e.g. the timestamps have changed.
  * I_DIRTY_PAGES	Inode has dirty pages.  Inode itself may be clean.
+ * I_DIRTY_TIME		The inode itself only has dirty timestamps, and the
+ *			lazytime mount option is enabled.  We keep track of this
+ *			separately from I_DIRTY_SYNC in order to implement
+ *			lazytime.  This gets cleared if I_DIRTY_INODE
+ *			(I_DIRTY_SYNC and/or I_DIRTY_DATASYNC) gets set.  I.e.
+ *			either I_DIRTY_TIME *or* I_DIRTY_INODE can be set in
+ *			i_state, but not both.  I_DIRTY_PAGES may still be set.
  * I_NEW		Serves as both a mutex and completion notification.
  *			New inodes set I_NEW.  If two processes both create
  *			the same inode, one of them will release its inode and