vfs: document ->d_real()

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

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 75eea7c..898d3ce 100644 (file)
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -20,6 +20,8 @@ prototypes:
        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 *, const struct inode *,
+                                unsigned int);
 
 locking rules:
                rename_lock     ->d_lock        may block       rcu-walk
@@ -34,6 +36,7 @@ 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:

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index c61a223..1c934dd 100644 (file)
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -938,6 +938,8 @@ 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 *, const struct inode *,
+                                unsigned int);
 };
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
@@ -1060,6 +1062,24 @@ 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)

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 45b22de..14df836 100644 (file)
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -139,7 +139,8 @@ 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 *, const struct inode *, unsigned int);
+       struct dentry *(*d_real)(struct dentry *, const struct inode *,
+                                unsigned int);
 } ____cacheline_aligned;
 
 /*
@@ -554,6 +555,17 @@ static inline struct dentry *d_backing_dentry(struct dentry *upper)
        return upper;
 }
 
+/**
+ * d_real - Return the real dentry
+ * @dentry: the dentry to query
+ * @inode: inode to select the dentry from multiple layers (can be NULL)
+ * @flags: open flags to control copy-up behavior
+ *
+ * If dentry is on an union/overlay, then return the underlying, real dentry.
+ * Otherwise return the dentry itself.
+ *
+ * See also: Documentation/filesystems/vfs.txt
+ */
 static inline struct dentry *d_real(struct dentry *dentry,
                                    const struct inode *inode,
                                    unsigned int flags)