[PATCH] new helpers - collect_mounts() and release_collected_mounts()

Get a snapshot of a subtree, creating private clones of vfsmounts
for all its components and release such snapshot resp.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -246,7 +246,7 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 			list_add(&mnt->mnt_slave, &old->mnt_slave_list);
 			mnt->mnt_master = old;
 			CLEAR_MNT_SHARED(mnt);
-		} else {
+		} else if (!(flag & CL_PRIVATE)) {
 			if ((flag & CL_PROPAGATION) || IS_MNT_SHARED(old))
 				list_add(&mnt->mnt_share, &old->mnt_share);
 			if (IS_MNT_SLAVE(old))
@@ -746,6 +746,26 @@ struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
 	return NULL;
 }
 
+struct vfsmount *collect_mounts(struct vfsmount *mnt, struct dentry *dentry)
+{
+	struct vfsmount *tree;
+	down_read(&namespace_sem);
+	tree = copy_tree(mnt, dentry, CL_COPY_ALL | CL_PRIVATE);
+	up_read(&namespace_sem);
+	return tree;
+}
+
+void drop_collected_mounts(struct vfsmount *mnt)
+{
+	LIST_HEAD(umount_list);
+	down_read(&namespace_sem);
+	spin_lock(&vfsmount_lock);
+	umount_tree(mnt, 0, &umount_list);
+	spin_unlock(&vfsmount_lock);
+	up_read(&namespace_sem);
+	release_mounts(&umount_list);
+}
+
 /*
  *  @source_mnt : mount tree to be attached
  *  @nd         : place the mount tree @source_mnt is attached

fs/pnode.h

@@ -22,6 +22,7 @@
 #define CL_COPY_ALL 		0x04
 #define CL_MAKE_SHARED 		0x08
 #define CL_PROPAGATION 		0x10
+#define CL_PRIVATE 		0x20
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {

include/linux/fs.h

@@ -1470,6 +1470,8 @@ extern long do_mount(char *, char *, char *, unsigned long, void *);
 extern struct vfsmount *copy_tree(struct vfsmount *, struct dentry *, int);
 extern void mnt_set_mountpoint(struct vfsmount *, struct dentry *,
 				  struct vfsmount *);
+extern struct vfsmount *collect_mounts(struct vfsmount *, struct dentry *);
+extern void drop_collected_mounts(struct vfsmount *);
 
 extern int vfs_statfs(struct dentry *, struct kstatfs *);