Index: sys/kern/vfs_mount.c
===================================================================
--- sys/kern/vfs_mount.c
+++ sys/kern/vfs_mount.c
@@ -546,6 +546,31 @@
 	uma_zfree(mount_zone, mp);
 }
 
+static bool
+vfs_should_downgrade_to_ro_mount(uint64_t fsflags, int error)
+{
+	/* This is an upgrade of an exisiting mount. */
+	if ((fsflags & MNT_UPDATE) != 0)
+		return (false);
+	/* This is already an R/O mount. */
+	if ((fsflags & MNT_RDONLY) != 0)
+		return (false);
+
+	switch (error) {
+	case ENODEV:	/* generic, geom, ... */
+	case EACCES:	/* cam/scsi, ... */
+	case EROFS:	/* md, mmcsd, ... */
+		/*
+		 * These errors can be returned by the storage layer to signal
+		 * that the media is read-only.  No harm in the R/O mount
+		 * attempt if the error is returned for some other reason.
+		 */
+		return (true);
+	default:
+		return (false);
+	}
+}
+
 int
 vfs_donmount(struct thread *td, uint64_t fsflags, struct uio *fsoptions)
 {
@@ -553,10 +578,12 @@
 	struct vfsopt *opt, *tmp_opt;
 	char *fstype, *fspath, *errmsg;
 	int error, fstypelen, fspathlen, errmsg_len, errmsg_pos;
+	int force_rw;
 
 	errmsg = fspath = NULL;
 	errmsg_len = fspathlen = 0;
 	errmsg_pos = -1;
+	force_rw = 0;
 
 	error = vfs_buildopts(fsoptions, &optlist);
 	if (error)
@@ -648,16 +675,23 @@
 			free(opt->name, M_MOUNT);
 			opt->name = strdup("nonosymfollow", M_MOUNT);
 		}
-		else if (strcmp(opt->name, "noro") == 0)
+		else if (strcmp(opt->name, "noro") == 0) {
 			fsflags &= ~MNT_RDONLY;
-		else if (strcmp(opt->name, "rw") == 0)
+			force_rw = 1;
+		}
+		else if (strcmp(opt->name, "rw") == 0) {
 			fsflags &= ~MNT_RDONLY;
-		else if (strcmp(opt->name, "ro") == 0)
+			force_rw = 1;
+		}
+		else if (strcmp(opt->name, "ro") == 0) {
 			fsflags |= MNT_RDONLY;
+			force_rw = 0;
+		}
 		else if (strcmp(opt->name, "rdonly") == 0) {
 			free(opt->name, M_MOUNT);
 			opt->name = strdup("ro", M_MOUNT);
 			fsflags |= MNT_RDONLY;
+			force_rw = 0;
 		}
 		else if (strcmp(opt->name, "suiddir") == 0)
 			fsflags |= MNT_SUIDDIR;
@@ -682,6 +716,18 @@
 	}
 
 	error = vfs_domount(td, fstype, fspath, fsflags, &optlist);
+
+	/*
+	 * See if we can mount in the read-only mode if the error code suggests
+	 * that it could be possible and the mount options allow for that.
+	 * Never try it if 'rw' or 'noro' have been explicitly requested.
+	 */
+	if (!force_rw && vfs_should_downgrade_to_ro_mount(fsflags, error)) {
+		printf("%s: R/W mount failed, possibly R/O media,"
+		    " trying R/O mount\n", __func__);
+		fsflags |= MNT_RDONLY;
+		error = vfs_domount(td, fstype, fspath, fsflags, &optlist);
+	}
 bail:
 	/* copyout the errmsg */
 	if (errmsg_pos != -1 && ((2 * errmsg_pos + 1) < fsoptions->uio_iovcnt)