o Fix module ref counts so that you can actually unload dm-mod

N.B. This means that you have to take very great care in the event that
   you want to access the dcache tree from in kernel

 o Added extra field to allow out of memory conditions to result in the
   correct error code. (This hasn't received a lot of testing...)

I've ditched the final project (which would have cleared my whole list)
since its got other complications which I don't have time to fix right
now. Still as Meatloaf says, two out of three ain't bad!

driver/device-mapper/dmfs-error.c

@@ -33,6 +33,19 @@ struct dmfs_error {
 	char *msg;
 };
 
+static struct dmfs_error oom_error;
+
+static struct list_head oom_list = {
+	next: &oom_error.list,
+	prev: &oom_error.list,
+};
+
+static struct dmfs_error oom_error = {
+	list: { next: &oom_list, prev: &oom_list },
+	len: 39,
+	msg: "Out of memory during creation of table\n",
+};
+
 void dmfs_add_error(struct inode *inode, unsigned num, char *str)
 {
 	struct dmfs_i *dmi = DMFS_I(inode);
@@ -64,9 +77,16 @@ static void *e_start(void *context, loff_t *pos)
 	struct dmfs_i *dmi = context;
 
 	down(&dmi->sem);
-	list_for_each(p, &dmi->errors)
-		if (n-- == 0)
-			return list_entry(p, struct dmfs_error, list);
+	if (dmi->status) {
+		list_for_each(p, &oom_list)
+			if (n-- == 0)
+				return list_entry(p, struct dmfs_error, list);
+	} else {
+		list_for_each(p, &dmi->errors)
+			if (n-- == 0)
+				return list_entry(p, struct dmfs_error, list);
+	}
+
 	return NULL;
 }
 
@@ -75,7 +95,7 @@ static void *e_next(void *context, void *v, loff_t *pos)
 	struct dmfs_i *dmi = context;
 	struct list_head *p = ((struct dmfs_error *)v)->list.next;
 	(*pos)++;
-	return (p == &dmi->errors) ? NULL 
+	return (p == &dmi->errors) || (p == &oom_list) ? NULL 
 				   : list_entry(p, struct dmfs_error, list);
 }
 

driver/device-mapper/dmfs-super.c

@@ -53,6 +53,7 @@ static void dmfs_delete_inode(struct inode *inode)
 		if (!list_empty(&dmi->errors))
 			dmfs_zap_errors(inode);
 		kfree(dmi);
+		MOD_DEC_USE_COUNT; /* Don't remove */
 	}
 
 	inode->u.generic_ip = NULL;
@@ -121,6 +122,7 @@ struct inode *dmfs_new_private_inode(struct super_block *sb, int mode)
 		init_MUTEX(&dmi->sem);
 		INIT_LIST_HEAD(&dmi->errors);
 		inode->u.generic_ip = dmi;
+		MOD_INC_USE_COUNT; /* Don't remove */
 	}
 	return inode;
 }
@@ -140,6 +142,8 @@ int __init dmfs_init(void)
 	if (IS_ERR(dmfs_mnt)) {
 		ret = PTR_ERR(dmfs_mnt);
 		unregister_filesystem(&dmfs_fstype);
+	} else {
+		MOD_DEC_USE_COUNT; /* Yes, this really is correct... */
 	}
 out:
 	return ret;
@@ -147,7 +151,9 @@ out:
 
 int __exit dmfs_exit(void)
 {
-	/* kern_umount(&dmfs_mnt); */
+	MOD_INC_USE_COUNT; /* So that it lands up being zero */
+
+	do_umount(dmfs_mnt, 0);
 
 	unregister_filesystem(&dmfs_fstype);
 

driver/device-mapper/dmfs.h

@@ -7,6 +7,7 @@ struct dmfs_i {
         struct mapped_device *md;
         struct dentry *dentry;
         struct list_head errors;
+	int status;
 };
 
 #define DMFS_I(inode) ((struct dmfs_i *)(inode)->u.generic_ip)

driver/device-mapper/projects.txt

@@ -1,15 +1,6 @@
     List of projects, ideas and pending bug fixes
    ===============================================
 
- o Need to work out how to make module use counts work correctly. The existing
-   scheme does not allow us to unload the module. A reference is taken during
-   kern_mount() which does not allow us to rmmod() so even if the right
-   umount() functions were exported (which they aren't) we'd never get to
-   call them anyway. This could be tricky.
-
  o Check tables are an integer multiple of the underlying block size at
    table load time.
 
- o Error code (as well as error list) so that "out of memory" errors can
-   result in messages which can be read from the error file.
-