diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 5303b3d..35ac504 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -3959,6 +3959,25 @@
 			return error;
 		}
 
+		/*
+		 * Version 5 superblock log feature mask validation. We know the
+		 * log is dirty so check if there are any unknown log features
+		 * in what we need to recover. If there are unknown features
+		 * (e.g. unsupported transactions, then simply reject the
+		 * attempt at recovery before touching anything.
+		 */
+		if (XFS_SB_VERSION_NUM(&log->l_mp->m_sb) == XFS_SB_VERSION_5 &&
+		    xfs_sb_has_incompat_log_feature(&log->l_mp->m_sb,
+					XFS_SB_FEAT_INCOMPAT_LOG_UNKNOWN)) {
+			xfs_warn(log->l_mp,
+"Superblock has unknown incompatible log features (0x%x) enabled.\n"
+"The log can not be fully and/or safely recovered by this kernel.\n"
+"Please recover the log on a kernel that supports the unknown features.",
+				(log->l_mp->m_sb.sb_features_log_incompat &
+					XFS_SB_FEAT_INCOMPAT_LOG_UNKNOWN));
+			return EINVAL;
+		}
+
 		xfs_notice(log->l_mp, "Starting recovery (logdev: %s)",
 				log->l_mp->m_logname ? log->l_mp->m_logname
 						     : "internal");
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 140136c..f6bfbd7 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -114,7 +114,9 @@
     { offsetof(xfs_sb_t, sb_features_compat), 0 },
     { offsetof(xfs_sb_t, sb_features_ro_compat), 0 },
     { offsetof(xfs_sb_t, sb_features_incompat), 0 },
+    { offsetof(xfs_sb_t, sb_features_log_incompat), 0 },
     { offsetof(xfs_sb_t, sb_crc),	 0 },
+    { offsetof(xfs_sb_t, sb_pad),	 0 },
     { offsetof(xfs_sb_t, sb_pquotino),	 0 },
     { offsetof(xfs_sb_t, sb_lsn),	 0 },
     { sizeof(xfs_sb_t),			 0 }
@@ -334,14 +336,45 @@
 	}
 
 	/*
-	 * Do not allow Version 5 superblocks to mount right now, even though
-	 * support is in place. We need to implement the proper feature masks
-	 * first.
+	 * Version 5 superblock feature mask validation. Reject combinations the
+	 * kernel cannot support up front before checking anything else.
 	 */
 	if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) {
 		xfs_alert(mp,
-	"Version 5 superblock detected. Experimental support not yet enabled!");
-		return XFS_ERROR(EINVAL);
+"Version 5 superblock detected. This kernel has EXPERIMENTAL support enabled!\n"
+"Use of these features in this kernel is at your own risk!");
+
+		if (xfs_sb_has_compat_feature(sbp,
+					XFS_SB_FEAT_COMPAT_UNKNOWN)) {
+			xfs_warn(mp,
+"Superblock has unknown compatible features (0x%x) enabled.\n"
+"Using a more recent kernel is recommended.",
+				(sbp->sb_features_compat &
+						XFS_SB_FEAT_COMPAT_UNKNOWN));
+		}
+
+		if (xfs_sb_has_ro_compat_feature(sbp,
+					XFS_SB_FEAT_RO_COMPAT_UNKNOWN)) {
+			xfs_alert(mp,
+"Superblock has unknown read-only compatible features (0x%x) enabled.",
+				(sbp->sb_features_ro_compat &
+						XFS_SB_FEAT_RO_COMPAT_UNKNOWN));
+			if (!(mp->m_flags & XFS_MOUNT_RDONLY)) {
+				xfs_warn(mp,
+"Attempted to mount read-only compatible filesystem read-write.\n"
+"Filesystem can only be safely mounted read only.");
+				return XFS_ERROR(EINVAL);
+			}
+		}
+		if (xfs_sb_has_incompat_feature(sbp,
+					XFS_SB_FEAT_INCOMPAT_UNKNOWN)) {
+			xfs_warn(mp,
+"Superblock has unknown incompatible features (0x%x) enabled.\n"
+"Filesystem can not be safely mounted by this kernel.",
+				(sbp->sb_features_incompat &
+						XFS_SB_FEAT_INCOMPAT_UNKNOWN));
+			return XFS_ERROR(EINVAL);
+		}
 	}
 
 	if (unlikely(
@@ -580,6 +613,9 @@
 	to->sb_features_compat = be32_to_cpu(from->sb_features_compat);
 	to->sb_features_ro_compat = be32_to_cpu(from->sb_features_ro_compat);
 	to->sb_features_incompat = be32_to_cpu(from->sb_features_incompat);
+	to->sb_features_log_incompat =
+				be32_to_cpu(from->sb_features_log_incompat);
+	to->sb_pad = 0;
 	to->sb_pquotino = be64_to_cpu(from->sb_pquotino);
 	to->sb_lsn = be64_to_cpu(from->sb_lsn);
 }
@@ -786,7 +822,7 @@
 	if (bp->b_error) {
 		error = bp->b_error;
 		if (loud)
-			xfs_warn(mp, "SB validate failed");
+			xfs_warn(mp, "SB validate failed with error %d.", error);
 		goto release_buf;
 	}
 
diff --git a/fs/xfs/xfs_sb.h b/fs/xfs/xfs_sb.h
index 457fefa..2de58a8 100644
--- a/fs/xfs/xfs_sb.h
+++ b/fs/xfs/xfs_sb.h
@@ -168,8 +168,10 @@
 	__uint32_t	sb_features_compat;
 	__uint32_t	sb_features_ro_compat;
 	__uint32_t	sb_features_incompat;
+	__uint32_t	sb_features_log_incompat;
 
 	__uint32_t	sb_crc;		/* superblock crc */
+	__uint32_t	sb_pad;
 
 	xfs_ino_t	sb_pquotino;	/* project quota inode */
 	xfs_lsn_t	sb_lsn;		/* last write sequence */
@@ -250,8 +252,10 @@
 	__be32		sb_features_compat;
 	__be32		sb_features_ro_compat;
 	__be32		sb_features_incompat;
+	__be32		sb_features_log_incompat;
 
 	__le32		sb_crc;		/* superblock crc */
+	__be32		sb_pad;
 
 	__be64		sb_pquotino;	/* project quota inode */
 	__be64		sb_lsn;		/* last write sequence */
@@ -276,7 +280,8 @@
 	XFS_SBS_INOALIGNMT, XFS_SBS_UNIT, XFS_SBS_WIDTH, XFS_SBS_DIRBLKLOG,
 	XFS_SBS_LOGSECTLOG, XFS_SBS_LOGSECTSIZE, XFS_SBS_LOGSUNIT,
 	XFS_SBS_FEATURES2, XFS_SBS_BAD_FEATURES2, XFS_SBS_FEATURES_COMPAT,
-	XFS_SBS_FEATURES_RO_COMPAT, XFS_SBS_FEATURES_INCOMPAT, XFS_SBS_CRC,
+	XFS_SBS_FEATURES_RO_COMPAT, XFS_SBS_FEATURES_INCOMPAT,
+	XFS_SBS_FEATURES_LOG_INCOMPAT, XFS_SBS_CRC, XFS_SBS_PAD,
 	XFS_SBS_PQUOTINO, XFS_SBS_LSN,
 	XFS_SBS_FIELDCOUNT
 } xfs_sb_field_t;
@@ -306,6 +311,7 @@
 #define XFS_SB_FEATURES_COMPAT	XFS_SB_MVAL(FEATURES_COMPAT)
 #define XFS_SB_FEATURES_RO_COMPAT XFS_SB_MVAL(FEATURES_RO_COMPAT)
 #define XFS_SB_FEATURES_INCOMPAT XFS_SB_MVAL(FEATURES_INCOMPAT)
+#define XFS_SB_FEATURES_LOG_INCOMPAT XFS_SB_MVAL(FEATURES_LOG_INCOMPAT)
 #define XFS_SB_CRC		XFS_SB_MVAL(CRC)
 #define XFS_SB_PQUOTINO		XFS_SB_MVAL(PQUOTINO)
 #define	XFS_SB_NUM_BITS		((int)XFS_SBS_FIELDCOUNT)
@@ -316,7 +322,8 @@
 	 XFS_SB_QFLAGS | XFS_SB_SHARED_VN | XFS_SB_UNIT | XFS_SB_WIDTH | \
 	 XFS_SB_ICOUNT | XFS_SB_IFREE | XFS_SB_FDBLOCKS | XFS_SB_FEATURES2 | \
 	 XFS_SB_BAD_FEATURES2 | XFS_SB_FEATURES_COMPAT | \
-	 XFS_SB_FEATURES_RO_COMPAT | XFS_SB_FEATURES_INCOMPAT | XFS_SB_PQUOTINO)
+	 XFS_SB_FEATURES_RO_COMPAT | XFS_SB_FEATURES_INCOMPAT | \
+	 XFS_SB_FEATURES_LOG_INCOMPAT | XFS_SB_PQUOTINO)
 
 
 /*
@@ -552,6 +559,65 @@
 	return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5;
 }
 
+
+/*
+ * Extended v5 superblock feature masks. These are to be used for new v5
+ * superblock features only.
+ *
+ * Compat features are new features that old kernels will not notice or affect
+ * and so can mount read-write without issues.
+ *
+ * RO-Compat (read only) are features that old kernels can read but will break
+ * if they write. Hence only read-only mounts of such filesystems are allowed on
+ * kernels that don't support the feature bit.
+ *
+ * InCompat features are features which old kernels will not understand and so
+ * must not mount.
+ *
+ * Log-InCompat features are for changes to log formats or new transactions that
+ * can't be replayed on older kernels. The fields are set when the filesystem is
+ * mounted, and a clean unmount clears the fields.
+ */
+#define XFS_SB_FEAT_COMPAT_ALL 0
+#define XFS_SB_FEAT_COMPAT_UNKNOWN	~XFS_SB_FEAT_COMPAT_ALL
+static inline bool
+xfs_sb_has_compat_feature(
+	struct xfs_sb	*sbp,
+	__uint32_t	feature)
+{
+	return (sbp->sb_features_compat & feature) != 0;
+}
+
+#define XFS_SB_FEAT_RO_COMPAT_ALL 0
+#define XFS_SB_FEAT_RO_COMPAT_UNKNOWN	~XFS_SB_FEAT_RO_COMPAT_ALL
+static inline bool
+xfs_sb_has_ro_compat_feature(
+	struct xfs_sb	*sbp,
+	__uint32_t	feature)
+{
+	return (sbp->sb_features_ro_compat & feature) != 0;
+}
+
+#define XFS_SB_FEAT_INCOMPAT_ALL 0
+#define XFS_SB_FEAT_INCOMPAT_UNKNOWN	~XFS_SB_FEAT_INCOMPAT_ALL
+static inline bool
+xfs_sb_has_incompat_feature(
+	struct xfs_sb	*sbp,
+	__uint32_t	feature)
+{
+	return (sbp->sb_features_incompat & feature) != 0;
+}
+
+#define XFS_SB_FEAT_INCOMPAT_LOG_ALL 0
+#define XFS_SB_FEAT_INCOMPAT_LOG_UNKNOWN	~XFS_SB_FEAT_INCOMPAT_LOG_ALL
+static inline bool
+xfs_sb_has_incompat_log_feature(
+	struct xfs_sb	*sbp,
+	__uint32_t	feature)
+{
+	return (sbp->sb_features_log_incompat & feature) != 0;
+}
+
 /*
  * end of superblock version macros
  */