xfs: fix directory inode iolock lockdep false positive

The change to add the IO lock to protect the directory extent map
during readdir operations has cause lockdep to have a heart attack
as it now sees a different locking order on inodes w.r.t. the
mmap_sem because readdir has a different ordering to write().

Add a new lockdep class for directory inodes to avoid this false
positive.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_iops.c

@@ -48,6 +48,18 @@
 #include <linux/fiemap.h>
 #include <linux/slab.h>
 
+/*
+ * Directories have different lock order w.r.t. mmap_sem compared to regular
+ * files. This is due to readdir potentially triggering page faults on a user
+ * buffer inside filldir(), and this happens with the ilock on the directory
+ * held. For regular files, the lock order is the other way around - the
+ * mmap_sem is taken during the page fault, and then we lock the ilock to do
+ * block mapping. Hence we need a different class for the directory ilock so
+ * that lockdep can tell them apart.
+ */
+static struct lock_class_key xfs_nondir_ilock_class;
+static struct lock_class_key xfs_dir_ilock_class;
+
 static int
 xfs_initxattrs(
 	struct inode		*inode,
@@ -1191,6 +1203,7 @@ xfs_setup_inode(
 	xfs_diflags_to_iflags(inode, ip);
 
 	ip->d_ops = ip->i_mount->m_nondir_inode_ops;
+	lockdep_set_class(&ip->i_lock.mr_lock, &xfs_nondir_ilock_class);
 	switch (inode->i_mode & S_IFMT) {
 	case S_IFREG:
 		inode->i_op = &xfs_inode_operations;
@@ -1198,6 +1211,7 @@ xfs_setup_inode(
 		inode->i_mapping->a_ops = &xfs_address_space_operations;
 		break;
 	case S_IFDIR:
+		lockdep_set_class(&ip->i_lock.mr_lock, &xfs_dir_ilock_class);
 		if (xfs_sb_version_hasasciici(&XFS_M(inode->i_sb)->m_sb))
 			inode->i_op = &xfs_dir_ci_inode_operations;
 		else