Sorry for my late response, I missed your feedback somehow.
On 12/12/2012 12:15 PM, Miao Xie wrote:

On wed, 12 Dec 2012 11:23:00 +0800, Jeff Liu wrote:

quoted

Add a new ioctl BTRFS_FS_SETLABEL to change the label of a mounted filesystem.


Signed-off-by: Jie Liu <redacted>
Signed-off-by: Anand Jain <redacted>

---
 fs/btrfs/ioctl.c |   34 ++++++++++++++++++++++++++++++++++
 fs/btrfs/ioctl.h |    2 ++
 2 files changed, 36 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index b0a5e17..ebbf634 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c

@@ -3711,6 +3711,38 @@ static int btrfs_ioctl_get_fslabel(struct btrfs_root *root, void __user *arg)
 	return ret ? -EFAULT : 0;
 }
 
+static int btrfs_ioctl_set_fslabel(struct btrfs_root *root, void __user *arg)
+{
+	struct btrfs_super_block *super_block = root->fs_info->super_copy;
+	struct btrfs_trans_handle *trans;
+	char label[BTRFS_LABEL_SIZE];
+	int ret = 0;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (btrfs_root_readonly(root))
+		return -EROFS;

Since this label is a global parameter, we needn't check the root
is R/O or not.

The check we need is the write access of the fs, so we need call
	mnt_want_write_file()

Yes, you are right.

quoted

+
+	if (copy_from_user(label, arg, sizeof(label)))
+		return -EFAULT;
+
+	label[BTRFS_LABEL_SIZE - 1] = '\0';

I don't think setting the last byte to '\0' is a good way, because the user would be
very strange that the new label is truncated, but the program told him that it was done
successfully.

Good point, will revise it later.

Thanks,
-Jeff

quoted

+
+	mutex_lock(&root->fs_info->volume_mutex);
+	trans = btrfs_start_transaction(root, 1);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		goto out_unlock;
+	}
+	strcpy(super_block->label, label);
+	btrfs_end_transaction(trans, root);
+
+out_unlock:
+	mutex_unlock(&root->fs_info->volume_mutex);
+	return ret;
+}
+
 long btrfs_ioctl(struct file *file, unsigned int
 		cmd, unsigned long arg)
 {

@@ -3811,6 +3843,8 @@ long btrfs_ioctl(struct file *file, unsigned int
 		return btrfs_ioctl_qgroup_limit(root, argp);
 	case BTRFS_IOC_GET_FSLABEL:
 		return btrfs_ioctl_get_fslabel(root, argp);
+	case BTRFS_IOC_SET_FSLABEL:
+		return btrfs_ioctl_set_fslabel(root, argp);
 	}
 
 	return -ENOTTY;

diff --git a/fs/btrfs/ioctl.h b/fs/btrfs/ioctl.h
index 98f896f..cc4e657 100644
--- a/fs/btrfs/ioctl.h
+++ b/fs/btrfs/ioctl.h

@@ -455,4 +455,6 @@ struct btrfs_ioctl_send_args {
 				      struct btrfs_ioctl_get_dev_stats)
 #define BTRFS_IOC_GET_FSLABEL _IOR(BTRFS_IOCTL_MAGIC, 53, \
 				   char[BTRFS_LABEL_SIZE])
+#define BTRFS_IOC_SET_FSLABEL _IOW(BTRFS_IOCTL_MAGIC, 54, \
+				   char[BTRFS_LABEL_SIZE])
 #endif

number 50 has been reserved for this feature.

Thanks
Miao