btrfs: scan but don't register device on single device filesystem

After the commit 5f58d783fd78 ("btrfs: free device in btrfs_close_devices
for a single device filesystem") we unregister the device from the kernel
memory upon unmounting for a single device.

So, device registration that was performed before mounting if any is no
longer in the kernel memory.

However, in fact, note that device registration is unnecessary for a
single-device btrfs filesystem unless it's a seed device.

So for commands like 'btrfs device scan' or 'btrfs device ready' with a
non-seed single-device btrfs filesystem, they can return success just
after superblock verification and without the actual device scan.  When
'device scan --forget' is called on such device no error is returned.

The seed device must remain in the kernel memory to allow the sprout
device to mount without the need to specify the seed device explicitly.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 18 insertions, 5 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 79d3037880b8..55d308cd5de4 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -846,7 +846,7 @@ static int btrfs_parse_device_options(const char *options, blk_mode_t flags)
 				error = -ENOMEM;
 				goto out;
 			}
-			device = btrfs_scan_one_device(device_name, flags);
+			device = btrfs_scan_one_device(device_name, flags, false);
 			kfree(device_name);
 			if (IS_ERR(device)) {
 				error = PTR_ERR(device);
@@ -1432,7 +1432,12 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 		goto error_fs_info;
 	}
 
-	device = btrfs_scan_one_device(device_name, mode);
+	/*
+	 * With 'true' passed to btrfs_scan_one_device() (mount time) we expect
+	 * either a valid device or an error.
+	 */
+	device = btrfs_scan_one_device(device_name, mode, true);
+	ASSERT(device != NULL);
 	if (IS_ERR(device)) {
 		mutex_unlock(&uuid_mutex);
 		error = PTR_ERR(device);
@@ -2144,7 +2149,11 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 	switch (cmd) {
 	case BTRFS_IOC_SCAN_DEV:
 		mutex_lock(&uuid_mutex);
-		device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ);
+		/*
+		 * Scanning outside of mount can return NULL which would turn
+		 * into 0 error code.
+		 */
+		device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false);
 		ret = PTR_ERR_OR_ZERO(device);
 		mutex_unlock(&uuid_mutex);
 		break;
@@ -2158,8 +2167,12 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 		break;
 	case BTRFS_IOC_DEVICES_READY:
 		mutex_lock(&uuid_mutex);
-		device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ);
-		if (IS_ERR(device)) {
+		/*
+		 * Scanning outside of mount can return NULL which would turn
+		 * into 0 error code.
+		 */
+		device = btrfs_scan_one_device(vol->name, BLK_OPEN_READ, false);
+		if (IS_ERR_OR_NULL(device)) {
 			mutex_unlock(&uuid_mutex);
 			ret = PTR_ERR(device);
 			break;