[PATCH 2/3] vfs: document ->d_real()

[Miklos Szeredi]

Add missing documentation for the d_op->d_real() method and d_real()
helper.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 Documentation/filesystems/Locking |    2 ++
 Documentation/filesystems/vfs.txt |   19 +++++++++++++++++++
 include/linux/dcache.h            |    7 +++++++
 3 files changed, 28 insertions(+)

--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -20,6 +20,7 @@ be able to use diff(1).
 	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 	struct vfsmount *(*d_automount)(struct path *path);
 	int (*d_manage)(struct dentry *, bool);
+	struct dentry *(*d_real)(struct dentry *, struct inode *, unsigned int);
 
 locking rules:
 		rename_lock	->d_lock	may block	rcu-walk
@@ -34,6 +35,7 @@ d_iput:		no		no		yes		no
 d_dname:	no		no		no		no
 d_automount:	no		no		yes		no
 d_manage:	no		no		yes (ref-walk)	maybe
+d_real		no		no		yes 		no
 
 --------------------------- inode_operations --------------------------- 
 prototypes:
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -938,6 +938,7 @@ struct dentry_operations {
 	char *(*d_dname)(struct dentry *, char *, int);
 	struct vfsmount *(*d_automount)(struct path *);
 	int (*d_manage)(struct dentry *, bool);
+	struct dentry *(*d_real)(struct dentry *, struct inode *, unsigned int);
 };
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
@@ -1060,6 +1061,24 @@ struct dentry_operations {
 	This function is only used if DCACHE_MANAGE_TRANSIT is set on the
 	dentry being transited from.
 
+  d_real: overlay/union type filesystems implement this method to return one of
+	the underlying dentries hidden by the overlay.  It is used in three
+	different modes:
+
+	Called from open it may need to copy-up the file depending on the
+	supplied open flags.  This mode is selected with a non-zero flags
+	argument.  In this mode the d_real method can return an error.
+
+	Called from file_dentry() it returns the real dentry matching the inode
+	argument.  The real dentry may be from a lower layer already copied up,
+	but still referenced from the file.  This mode is selected with a
+	non-NULL inode argument.  This will always succeed.
+
+	With NULL inode and zero flags the topmost real underlying dentry is
+	returned.  This will always succeed.
+
+	This method is never called with both non-NULL inode and non-zero flags.
+
 Example :
 
 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -554,6 +554,13 @@ static inline struct dentry *d_backing_d
 	return upper;
 }
 
+/**
+ * d_real - Return the real dentry
+ * @dentry: The dentry to query
+ *
+ * If dentry is on an union/overlay, then return the underlying, real dentry.
+ * Otherwise return the dentry itself.
+ */
 static inline struct dentry *d_real(struct dentry *dentry)
 {
 	if (unlikely(dentry->d_flags & DCACHE_OP_REAL))