Fix snapdir check and add comments

fs/nfsd/vfs.c

@@ -112,15 +112,38 @@ nfserrno (int errno)
 	return nfserr_io;
 }
 
+/*
+ * Called from nfsd_cross_mnt and is used to determine
+ * whether we need to set LOOKUP_AUTOMOUNT flag.
+ *
+ * ZFSCTL_INO_SNAPDIR is defined in sys/zfs_ctldir.h
+ * and is unlikely to change. This is a hard-coded inode
+ * number for ./zfs/snapshot directory in the ZFS ctldir.
+ *
+ * If we know the parent inode number is the snapdir then
+ * we also know that the curent dentry is for an auto-
+ * mounted snapshot.
+ */
 static int
-is_zfs_snapdir(struct dentry *dentry)
+is_in_zfs_snapdir(struct dentry *dentry)
 {
-#define	ZFSCTL_INO_SNAPDIR 0x0000FFFFFFFFFFFDULL
+#define ZFSCTL_INO_SNAPDIR 0x0000FFFFFFFFFFFDULL
 
-	struct inode *inode = d_inode(dentry);
+	struct dentry *dp = dentry->d_parent;
+	struct inode *inode = NULL;
+
+	if (dp == NULL)
+		return 0;
+
+	inode = d_inode(dp);
 	if (inode == NULL)
 		return 0;
 
+	// Currently only ZFS has large xattr support enabled.
+	if (!IS_LARGE_XATTR(inode))
+		return 0;
+
+	// The ZFS snapdir has a hard-coded inode value
 	return (inode->i_ino == ZFSCTL_INO_SNAPDIR);
 }
 
@@ -145,9 +168,11 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 	if (exp->ex_flags & NFSEXP_CROSSMOUNT)
 		follow_flags = LOOKUP_AUTOMOUNT;
 
-	if ((exp->ex_flags & NFSEXP_SNAPDIR) && is_zfs_snapdir(dentry)) {
-		follow_flags = LOOKUP_AUTOMOUNT;
-		is_snapdir = 1;
+	// ZFS ctldir specific handling
+	if (exp->ex_flags & NFSEXP_SNAPDIR) {
+		is_snapdir = is_in_zfs_snapdir(dentry);
+		if (is_snapdir)
+			follow_flags = LOOKUP_AUTOMOUNT;
 	}
 
 	err = follow_down(&path, follow_flags);