btrfs: add ioctl to export size of global metadata reservation

btrfs filesystem df output will show the size of the metadata space
and how much of it is used, and the user assumes that the difference
is all usable space. Since that's not actually the case due to the
global metadata reservation, we should provide the full picture to the
user.

This patch adds an ioctl that exports the size of the global metadata
reservation so that btrfs filesystem df can report it.
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ioctl.c

@@ -3498,6 +3498,20 @@ static long btrfs_ioctl_space_info(struct btrfs_root *root, void __user *arg)
 	return ret;
 }
 
+static long btrfs_ioctl_global_rsv(struct btrfs_root *root, void __user *arg)
+{
+	struct btrfs_block_rsv *block_rsv = &root->fs_info->global_block_rsv;
+	u64 reserved;
+
+	spin_lock(&block_rsv->lock);
+	reserved = block_rsv->reserved;
+	spin_unlock(&block_rsv->lock);
+
+	if (arg && copy_to_user(arg, &reserved, sizeof(reserved)))
+		return -EFAULT;
+	return 0;
+}
+
 /*
  * there are many ways the trans_start and trans_end ioctls can lead
  * to deadlocks.  They should only be used by applications that
@@ -4706,6 +4720,8 @@ long btrfs_ioctl(struct file *file, unsigned int
 		return btrfs_ioctl_logical_to_ino(root, argp);
 	case BTRFS_IOC_SPACE_INFO:
 		return btrfs_ioctl_space_info(root, argp);
+	case BTRFS_IOC_GLOBAL_RSV:
+		return btrfs_ioctl_global_rsv(root, argp);
 	case BTRFS_IOC_SYNC: {
 		int ret;
 

include/uapi/linux/btrfs.h

@@ -558,6 +558,7 @@ static inline char *btrfs_err_str(enum btrfs_err_code err_code)
 #define BTRFS_IOC_DEFAULT_SUBVOL _IOW(BTRFS_IOCTL_MAGIC, 19, __u64)
 #define BTRFS_IOC_SPACE_INFO _IOWR(BTRFS_IOCTL_MAGIC, 20, \
 				    struct btrfs_ioctl_space_args)
+#define BTRFS_IOC_GLOBAL_RSV _IOR(BTRFS_IOCTL_MAGIC, 20, __u64)
 #define BTRFS_IOC_START_SYNC _IOR(BTRFS_IOCTL_MAGIC, 24, __u64)
 #define BTRFS_IOC_WAIT_SYNC  _IOW(BTRFS_IOCTL_MAGIC, 22, __u64)
 #define BTRFS_IOC_SNAP_CREATE_V2 _IOW(BTRFS_IOCTL_MAGIC, 23, \