btrfs: add extra error messages to cover non-ENOMEM errors from device_add_list()

[BUG]
When test case btrfs/219 (aka, mount a registered device but with a lower
generation) failed, there is not any useful information for the end user
to find out what's going wrong.

The mount failure just looks like this:

  #  mount -o loop /tmp/219.img2 /mnt/btrfs/
  mount: /mnt/btrfs: mount(2) system call failed: File exists.
         dmesg(1) may have more information after failed mount system call.

While the dmesg contains nothing but the loop device change:

  loop1: detected capacity change from 0 to 524288

[CAUSE]
In device_list_add() we have a lot of extra checks to reject invalid
cases.

That function also contains the regular device scan result like the
following prompt:

  BTRFS: device fsid 6222333e-f9f1-47e6-b306-55ddd4dcaef4 devid 1 transid 8 /dev/loop0 scanned by systemd-udevd (3027)

But unfortunately not all errors have their own error messages, thus if
we hit something wrong in device_add_list(), there may be no error
messages at all.

[FIX]
Add errors message for all non-ENOMEM errors.

For ENOMEM, I'd say we're in a much worse situation, and there should be
some OOM messages way before our call sites.

CC: [email protected] # 6.0+
Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adam900710

fs/btrfs/volumes.c

@@ -768,8 +768,11 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 					BTRFS_SUPER_FLAG_CHANGING_FSID_V2);
 
 	error = lookup_bdev(path, &path_devt);
-	if (error)
+	if (error) {
+		btrfs_err(NULL, "failed to lookup block device for path %s: %d",
+			  path, error);
 		return ERR_PTR(error);
+	}
 
 	if (fsid_change_in_progress) {
 		if (!has_metadata_uuid)
@@ -836,6 +839,9 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 		unsigned int nofs_flag;
 
 		if (fs_devices->opened) {
+			btrfs_err(NULL,
+		"device %s belongs to fsid %pU, and the fs is already mounted",
+				  path, fs_devices->fsid);
 			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-EBUSY);
 		}
@@ -905,6 +911,9 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 			 * generation are equal.
 			 */
 			mutex_unlock(&fs_devices->device_list_mutex);
+			btrfs_err(NULL,
+"device %s already registered with a higher generation, found %llu expect %llu",
+				  path, found_transid, device->generation);
 			return ERR_PTR(-EEXIST);
 		}