xfs: implement online get/set fs label

The GET ioctl is trivial, just return the current label.

The SET ioctl is more involved:
It transactionally modifies the superblock to write a new filesystem
label to the primary super.

A new variant of xfs_sync_sb then writes the superblock buffer
immediately to disk so that the change is visible from userspace.

It then invalidates any page cache that userspace might have previously
read on the block device so that i.e. blkid can see the change
immediately, and updates all secondary superblocks as userspace relable
does.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[darrick: use dchinner's new xfs_update_secondary_sbs function]
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

3 files changed, 36 insertions, 2 deletions

diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index 42956d8d95ed..c1cb29a5f4f6 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -98,6 +98,9 @@ struct xfs_ifork;
 	 XFS_SB_VERSION2_PROJID32BIT	| \
 	 XFS_SB_VERSION2_FTYPE)
 
+/* Maximum size of the xfs filesystem label, no terminating NULL */
+#define XFSLABEL_MAX			12
+
 /*
  * Superblock - in core version.  Must match the ondisk version below.
  * Must be padded to 64 bit alignment.
@@ -122,7 +125,7 @@ typedef struct xfs_sb {
 	uint16_t	sb_sectsize;	/* volume sector size, bytes */
 	uint16_t	sb_inodesize;	/* inode size, bytes */
 	uint16_t	sb_inopblock;	/* inodes per block */
-	char		sb_fname[12];	/* file system name */
+	char		sb_fname[XFSLABEL_MAX]; /* file system name */
 	uint8_t		sb_blocklog;	/* log2 of sb_blocksize */
 	uint8_t		sb_sectlog;	/* log2 of sb_sectsize */
 	uint8_t		sb_inodelog;	/* log2 of sb_inodesize */
@@ -213,7 +216,7 @@ typedef struct xfs_dsb {
 	__be16		sb_sectsize;	/* volume sector size, bytes */
 	__be16		sb_inodesize;	/* inode size, bytes */
 	__be16		sb_inopblock;	/* inodes per block */
-	char		sb_fname[12];	/* file system name */
+	char		sb_fname[XFSLABEL_MAX]; /* file system name */
 	__u8		sb_blocklog;	/* log2 of sb_blocksize */
 	__u8		sb_sectlog;	/* log2 of sb_sectsize */
 	__u8		sb_inodelog;	/* log2 of sb_inodesize */
diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
index a35344ae4951..ea6c85a4c27c 100644
--- a/fs/xfs/libxfs/xfs_sb.c
+++ b/fs/xfs/libxfs/xfs_sb.c
@@ -961,6 +961,36 @@ xfs_update_secondary_sbs(
 	return saved_error ? saved_error : error;
 }
 
+/*
+ * Same behavior as xfs_sync_sb, except that it is always synchronous and it
+ * also writes the superblock buffer to disk sector 0 immediately.
+ */
+int
+xfs_sync_sb_buf(
+	struct xfs_mount	*mp)
+{
+	struct xfs_trans	*tp;
+	int			error;
+
+	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_sb, 0, 0, 0, &tp);
+	if (error)
+		return error;
+
+	xfs_log_sb(tp);
+	xfs_trans_bhold(tp, mp->m_sb_bp);
+	xfs_trans_set_sync(tp);
+	error = xfs_trans_commit(tp);
+	if (error)
+		goto out;
+	/*
+	 * write out the sb buffer to get the changes to disk
+	 */
+	error = xfs_bwrite(mp->m_sb_bp);
+out:
+	xfs_buf_relse(mp->m_sb_bp);
+	return error;
+}
+
 int
 xfs_fs_geometry(
 	struct xfs_sb		*sbp,
diff --git a/fs/xfs/libxfs/xfs_sb.h b/fs/xfs/libxfs/xfs_sb.h
index fc65d4e1433a..03e0cc6bf3a6 100644
--- a/fs/xfs/libxfs/xfs_sb.h
+++ b/fs/xfs/libxfs/xfs_sb.h
@@ -36,6 +36,7 @@ extern int	xfs_initialize_perag_data(struct xfs_mount *, xfs_agnumber_t);
 
 extern void	xfs_log_sb(struct xfs_trans *tp);
 extern int	xfs_sync_sb(struct xfs_mount *mp, bool wait);
+extern int	xfs_sync_sb_buf(struct xfs_mount *mp);
 extern void	xfs_sb_mount_common(struct xfs_mount *mp, struct xfs_sb *sbp);
 extern void	xfs_sb_from_disk(struct xfs_sb *to, struct xfs_dsb *from);
 extern void	xfs_sb_to_disk(struct xfs_dsb *to, struct xfs_sb *from);