[PATCH] Add explanation of I_DIRTY_DATASYNC bit

[Jan Kara]

Add explanation of I_DIRTY_DATASYNC bit.

Signed-off-by: Jan Kara <jack@xxxxxxx>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 56bd421..475125e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1280,7 +1280,10 @@ 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_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.
  * 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 +1315,6 @@ 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?
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