[PATCH 15/34] union-mount: Add union_create_topmost_dir()

[Valerie Aurora]

Union mounts design requires that the topmost directory exist for
every single directory at the time lookup completes.  This is so that
we don't have to double back and create a whole path's worth of
directories whenever we copy up a file in a directory for the first
time.  This greatly simplifies locking and error handling.

XXX - attributes of copied-up dir are wrong

Signed-off-by: Valerie Aurora <vaurora@xxxxxxxxxx>
---
 fs/union.c |   34 ++++++++++++++++++++++++++++++++++
 fs/union.h |    3 +++
 2 files changed, 37 insertions(+), 0 deletions(-)

diff --git a/fs/union.c b/fs/union.c
index 45552f8..bc53066 100644
--- a/fs/union.c
+++ b/fs/union.c
@@ -93,3 +93,37 @@ int union_add_dir(struct path *topmost, struct path *lower,
 	*path = *lower;
 	return 0;
 }
+
+/**
+ * union_create_topmost_dir - Create a matching dir in the topmost file system
+ *
+ * @parent - parent of target on topmost layer
+ * @name - name of target
+ * @topmost - path of target on topmost layer
+ * @lower - path of source on lower layer
+ *
+ * As we lookup each directory on the lower layer of a union, we
+ * create a matching directory on the topmost layer if it does not
+ * already exist.
+ *
+ * XXX - owner is wrong, set credentials properly
+ */
+
+int union_create_topmost_dir(struct path *parent, struct qstr *name,
+			     struct path *topmost, struct path *lower)
+{
+	int mode = lower->dentry->d_inode->i_mode;
+	int res;
+
+	BUG_ON(topmost->dentry->d_inode);
+
+	res = mnt_want_write(parent->mnt);
+	if (res)
+		return res;
+
+	res = vfs_mkdir(parent->dentry->d_inode, topmost->dentry, mode);
+
+	mnt_drop_write(parent->mnt);
+
+	return res;
+}
diff --git a/fs/union.h b/fs/union.h
index bd03d67..1692803 100644
--- a/fs/union.h
+++ b/fs/union.h
@@ -53,6 +53,8 @@ struct union_stack {
 
 extern void d_free_unions(struct dentry *);
 extern int union_add_dir(struct path *, struct path *, unsigned int);
+extern int union_create_topmost_dir(struct path *, struct qstr *, struct path *,
+				    struct path *);
 
 static inline struct path *union_find_dir(struct dentry *dentry,
 					  unsigned int layer) {
@@ -67,6 +69,7 @@ static inline struct path *union_find_dir(struct dentry *dentry,
 #define d_free_unions(x)		do { } while (0)
 #define union_add_dir(x, y, z)		({ BUG(); (0); })
 #define union_find_dir(x, y)		({ BUG(); (NULL); })
+#define union_create_topmost_dir(w, x, y, z)	({ BUG(); (0); })
 
 #endif	/* CONFIG_UNION_MOUNT */
 #endif	/* __KERNEL__ */
-- 
1.6.3.3

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