[PATCH v2 11/15] xfs: mount api - add xfs_reconfigure()

[Ian Kent <raven@themaw.net>]

Add the fs_context_operations method .reconfigure that performs
remount validation as previously done by the super_operations
.remount_fs method.

An attempt has also been made to update the comment about options
handling problems with mount(8) to reflect the current situation.

Signed-off-by: Ian Kent <raven@themaw.net>
---
 fs/xfs/xfs_super.c |   84 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 76374d602257..aae0098fecab 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1522,6 +1522,89 @@ xfs_fs_remount(
 	return 0;
 }
 
+/*
+ * There have been problems in the past with options passed from mount(8).
+ *
+ * The problem being that options passed by mount(8) in the case where only
+ * the the mount point path is given would consist of the existing fstab
+ * options with the options from mtab for the current mount merged in and
+ * the options given on the command line last. But the result couldn't be
+ * relied upon to accurately reflect the current mount options so that
+ * rejecting options that can't be changed on reconfigure could erronously
+ * cause mount failure.
+ *
+ * The mount-api uses a legacy mount options handler in the VFS to handle
+ * mount(8) so these options will continue to be passed. Even if mount(8)
+ * is updated to use fsopen()/fsconfig()/fsmount() it's likely to continue
+ * to set the existing options so options problems with reconfigure could
+ * continue.
+ *
+ * For the longest time mtab locking was a problem and this could have been
+ * one possible cause. It's also possible there could have been options
+ * order problems.
+ *
+ * That has changed now as mtab is a link to the proc file system mount
+ * table so mtab options should be always accurate.
+ *
+ * Consulting the util-linux maintainer (Karel Zak) he is confident that,
+ * in this case, the options passed by mount(8) will be those of the current
+ * mount and the options order should be a correct merge of fstab and mtab
+ * options, and new options given on the command line.
+ *
+ * So, in theory, it should be possible to compare incoming options and
+ * return an error for options that differ from the current mount and can't
+ * be changed on reconfigure to prevent users from believing they might have
+ * changed mount options using remount which can't be changed.
+ *
+ * But for now continue to return success for every reconfigure request, and
+ * silently ignore all options that can't actually be changed.
+ */
+STATIC int
+xfs_reconfigure(
+	struct fs_context *fc)
+{
+	struct xfs_fs_context	*ctx = fc->fs_private;
+	struct xfs_mount	*mp = XFS_M(fc->root->d_sb);
+	struct xfs_mount        *new_mp = fc->s_fs_info;
+	xfs_sb_t		*sbp = &mp->m_sb;
+	int			flags = fc->sb_flags;
+	int			error;
+
+	error = xfs_validate_params(new_mp, ctx, false);
+	if (error)
+		return error;
+
+	/* inode32 -> inode64 */
+	if ((mp->m_flags & XFS_MOUNT_SMALL_INUMS) &&
+	    !(new_mp->m_flags & XFS_MOUNT_SMALL_INUMS)) {
+		mp->m_flags &= ~XFS_MOUNT_SMALL_INUMS;
+		mp->m_maxagi = xfs_set_inode_alloc(mp, sbp->sb_agcount);
+	}
+
+	/* inode64 -> inode32 */
+	if (!(mp->m_flags & XFS_MOUNT_SMALL_INUMS) &&
+	    (new_mp->m_flags & XFS_MOUNT_SMALL_INUMS)) {
+		mp->m_flags |= XFS_MOUNT_SMALL_INUMS;
+		mp->m_maxagi = xfs_set_inode_alloc(mp, sbp->sb_agcount);
+	}
+
+	/* ro -> rw */
+	if ((mp->m_flags & XFS_MOUNT_RDONLY) && !(flags & SB_RDONLY)) {
+		error = xfs_remount_rw(mp);
+		if (error)
+			return error;
+	}
+
+	/* rw -> ro */
+	if (!(mp->m_flags & XFS_MOUNT_RDONLY) && (flags & SB_RDONLY)) {
+		error = xfs_remount_ro(mp);
+		if (error)
+			return error;
+	}
+
+	return 0;
+}
+
 /*
  * Second stage of a freeze. The data is already frozen so we only
  * need to take care of the metadata. Once that's done sync the superblock
@@ -2049,6 +2132,7 @@ static const struct super_operations xfs_super_operations = {
 static const struct fs_context_operations xfs_context_ops = {
 	.parse_param = xfs_parse_param,
 	.get_tree    = xfs_get_tree,
+	.reconfigure = xfs_reconfigure,
 };
 
 static struct file_system_type xfs_fs_type = {