diff --git a/e2fsck/e2fsck.h b/e2fsck/e2fsck.h
index d4b472f5..f46a95ef 100644
--- a/e2fsck/e2fsck.h
+++ b/e2fsck/e2fsck.h
@@ -450,6 +450,7 @@ struct e2fsck_struct {
 	/* Undo file */
 	char *undo_file;
 #ifdef CONFIG_PFSCK
+	__u32			 fs_num_threads;
 	/* serialize fix operation for multiple threads */
 	pthread_mutex_t		 fs_fix_mutex;
 	/* protect block_found_map, block_dup_map */
@@ -684,6 +685,7 @@ int check_backup_super_block(e2fsck_t ctx);
 void check_resize_inode(e2fsck_t ctx);
 
 /* util.c */
+#define E2FSCK_MAX_THREADS	(65536)
 extern void *e2fsck_allocate_memory(e2fsck_t ctx, unsigned long size,
 				    const char *description);
 extern int ask(e2fsck_t ctx, const char * string, int def);
diff --git a/e2fsck/pass1.c b/e2fsck/pass1.c
index 6dba6d1b..30365d23 100644
--- a/e2fsck/pass1.c
+++ b/e2fsck/pass1.c
@@ -1194,6 +1194,97 @@ static int e2fsck_should_abort(e2fsck_t ctx)
 	return 0;
 }
 
+static void init_ext2_max_sizes()
+{
+	int	i;
+	__u64	max_sizes;
+
+	/*
+	 * Init ext2_max_sizes which will be immutable and shared between
+	 * threads
+	 */
+#define EXT2_BPP(bits) (1ULL << ((bits) - 2))
+
+	for (i = EXT2_MIN_BLOCK_LOG_SIZE; i <= EXT2_MAX_BLOCK_LOG_SIZE; i++) {
+		max_sizes = EXT2_NDIR_BLOCKS + EXT2_BPP(i);
+		max_sizes = max_sizes + EXT2_BPP(i) * EXT2_BPP(i);
+		max_sizes = max_sizes + EXT2_BPP(i) * EXT2_BPP(i) * EXT2_BPP(i);
+		max_sizes = (max_sizes * (1UL << i));
+		ext2_max_sizes[i - EXT2_MIN_BLOCK_LOG_SIZE] = max_sizes;
+	}
+#undef EXT2_BPP
+}
+
+#ifdef CONFIG_PFSCK
+/* TODO: tdb needs to be handled properly for multiple threads*/
+static int multiple_threads_supported(e2fsck_t ctx)
+{
+#ifdef	CONFIG_TDB
+	unsigned int		threshold;
+	ext2_ino_t		num_dirs;
+	errcode_t		retval;
+	char			*tdb_dir;
+	int			enable;
+
+	profile_get_string(ctx->profile, "scratch_files", "directory", 0, 0,
+			   &tdb_dir);
+	profile_get_uint(ctx->profile, "scratch_files",
+			 "numdirs_threshold", 0, 0, &threshold);
+	profile_get_boolean(ctx->profile, "scratch_files",
+			    "icount", 0, 1, &enable);
+
+	retval = ext2fs_get_num_dirs(ctx->fs, &num_dirs);
+	if (retval)
+		num_dirs = 1024;	/* Guess */
+
+	/* tdb is unsupported now */
+	if (enable && tdb_dir && !access(tdb_dir, W_OK) &&
+	    (!threshold || num_dirs > threshold))
+		return 0;
+#endif
+	return 1;
+}
+
+/**
+ * Even though we could specify number of threads,
+ * but it might be more than the whole filesystem
+ * block groups, correct it here.
+ */
+static void e2fsck_pass1_set_thread_num(e2fsck_t ctx)
+{
+	unsigned flexbg_size = 1;
+	ext2_filsys fs = ctx->fs;
+	int num_threads = ctx->fs_num_threads;
+	int max_threads;
+
+	if (num_threads < 1) {
+		num_threads = 1;
+		goto out;
+	}
+
+	if (!multiple_threads_supported(ctx)) {
+		num_threads = 1;
+		fprintf(stderr, "Fall through single thread for pass1 "
+			"because tdb could not handle properly\n");
+		goto out;
+	}
+
+	if (ext2fs_has_feature_flex_bg(fs->super))
+		flexbg_size = 1 << fs->super->s_log_groups_per_flex;
+	max_threads = fs->group_desc_count / flexbg_size;
+	if (max_threads == 0)
+		max_threads = 1;
+
+	if (num_threads > max_threads) {
+		fprintf(stderr, "Use max possible thread num: %d instead\n",
+				max_threads);
+		num_threads = max_threads;
+	}
+out:
+	ctx->fs_num_threads = num_threads;
+}
+#endif
+
 /*
  * We need call mark_table_blocks() before multiple
  * thread start, since all known system blocks should be
@@ -1204,6 +1295,11 @@ static errcode_t e2fsck_pass1_prepare(e2fsck_t ctx)
 	struct problem_context pctx;
 	ext2_filsys fs = ctx->fs;
 
+	init_ext2_max_sizes();
+#ifdef	CONFIG_PFSCK
+	e2fsck_pass1_set_thread_num(ctx);
+#endif
+
 	clear_problem_context(&pctx);
 	if (!(ctx->options & E2F_OPT_PREEN))
 		fix_problem(ctx, PR_1_PASS_HEADER, &pctx);
@@ -2207,27 +2303,6 @@ endit:
 		ctx->invalid_bitmaps++;
 }
 
-static void init_ext2_max_sizes()
-{
-	int	i;
-	__u64	max_sizes;
-
-	/*
-	 * Init ext2_max_sizes which will be immutable and shared between
-	 * threads
-	 */
-#define EXT2_BPP(bits) (1ULL << ((bits) - 2))
-
-	for (i = EXT2_MIN_BLOCK_LOG_SIZE; i <= EXT2_MAX_BLOCK_LOG_SIZE; i++) {
-		max_sizes = EXT2_NDIR_BLOCKS + EXT2_BPP(i);
-		max_sizes = max_sizes + EXT2_BPP(i) * EXT2_BPP(i);
-		max_sizes = max_sizes + EXT2_BPP(i) * EXT2_BPP(i) * EXT2_BPP(i);
-		max_sizes = (max_sizes * (1UL << i));
-		ext2_max_sizes[i - EXT2_MIN_BLOCK_LOG_SIZE] = max_sizes;
-	}
-#undef EXT2_BPP
-}
-
 #ifdef CONFIG_PFSCK
 static errcode_t e2fsck_pass1_copy_bitmap(ext2_filsys fs, ext2fs_generic_bitmap *src,
 					  ext2fs_generic_bitmap *dest)
@@ -3220,9 +3295,9 @@ static int e2fsck_pass1_threads_start(struct e2fsck_thread_info **pinfo,
 
 static void e2fsck_pass1_multithread(e2fsck_t global_ctx)
 {
-	struct e2fsck_thread_info	*infos = NULL;
-	int				 num_threads = 1;
-	errcode_t			 retval;
+	struct e2fsck_thread_info *infos = NULL;
+	int num_threads = global_ctx->fs_num_threads;
+	errcode_t retval;
 
 	retval = e2fsck_pass1_threads_start(&infos, num_threads, global_ctx);
 	if (retval) {
@@ -3244,54 +3319,22 @@ out_abort:
 }
 #endif
 
-/* TODO: tdb needs to be handled properly for multiple threads*/
-static int multiple_threads_supported(e2fsck_t ctx)
-{
-#ifdef	CONFIG_TDB
-	unsigned int		threshold;
-	ext2_ino_t		num_dirs;
-	errcode_t		retval;
-	char			*tdb_dir;
-	int			enable;
-
-	profile_get_string(ctx->profile, "scratch_files", "directory", 0, 0,
-			   &tdb_dir);
-	profile_get_uint(ctx->profile, "scratch_files",
-			 "numdirs_threshold", 0, 0, &threshold);
-	profile_get_boolean(ctx->profile, "scratch_files",
-			    "icount", 0, 1, &enable);
-
-	retval = ext2fs_get_num_dirs(ctx->fs, &num_dirs);
-	if (retval)
-		num_dirs = 1024;	/* Guess */
-
-	/* tdb is unsupported now */
-	if (enable && tdb_dir && !access(tdb_dir, W_OK) &&
-	    (!threshold || num_dirs > threshold))
-		return 0;
- #endif
-	return 1;
-}
-
 void e2fsck_pass1(e2fsck_t ctx)
 {
 	errcode_t retval;
-	int multiple = 0;
+	int need_single = 1;
 
-	init_ext2_max_sizes();
 	retval = e2fsck_pass1_prepare(ctx);
 	if (retval)
 		return;
 #ifdef CONFIG_PFSCK
-	if (multiple_threads_supported(ctx)) {
-		multiple = 1;
+	if (ctx->fs_num_threads > 1 ||
+	    ctx->options & E2F_OPT_MULTITHREAD) {
+		need_single = 0;
 		e2fsck_pass1_multithread(ctx);
-	} else {
-		fprintf(stderr, "Fall through single thread for pass1 "
-				"because tdb could not handle properly\n");
 	}
 #endif
-	if (!multiple)
+	if (need_single)
 		e2fsck_pass1_run(ctx);
 	e2fsck_pass1_post(ctx);
 }
diff --git a/e2fsck/unix.c b/e2fsck/unix.c
index 30c2bf31..cd31bcd5 100644
--- a/e2fsck/unix.c
+++ b/e2fsck/unix.c
@@ -819,6 +819,10 @@ static errcode_t PRS(int argc, char *argv[], e2fsck_t *ret_ctx)
 	int 		res;		/* result of sscanf */
 #ifdef CONFIG_JBD_DEBUG
 	char 		*jbd_debug;
+#endif
+#ifdef CONFIG_PFSCK
+	char		*pm;
+	unsigned long	thread_num;
 #endif
 	unsigned long long phys_mem_kb;
 
@@ -852,7 +856,7 @@ static errcode_t PRS(int argc, char *argv[], e2fsck_t *ret_ctx)
 	ctx->readahead_kb = ~0ULL;
 
 #ifdef CONFIG_PFSCK
-	while ((c = getopt(argc, argv, "pamnyrcC:B:dE:fvtFVM:b:I:j:P:l:L:N:SsDkz:")) != EOF)
+	while ((c = getopt(argc, argv, "pam:nyrcC:B:dE:fvtFVM:b:I:j:P:l:L:N:SsDkz:")) != EOF)
 #else
 	while ((c = getopt(argc, argv, "panyrcC:B:dE:fvtFVM:b:I:j:P:l:L:N:SsDkz:")) != EOF)
 #endif
@@ -898,7 +902,18 @@ static errcode_t PRS(int argc, char *argv[], e2fsck_t *ret_ctx)
 			break;
 #ifdef CONFIG_PFSCK
 		case 'm':
+			thread_num = strtoul(optarg, &pm, 0);
+			if (*pm)
+				fatal_error(ctx,
+					_("Invalid multiple thread num.\n"));
+			if (thread_num > E2FSCK_MAX_THREADS) {
+				fprintf(stderr,
+					_("threads %lu too large (max %lu)\n"),
+					thread_num, E2FSCK_MAX_THREADS);
+				fatal_error(ctx, 0);
+			}
 			ctx->options |= E2F_OPT_MULTITHREAD;
+			ctx->fs_num_threads = thread_num;
 			break;
 #endif
 		case 'n':
diff --git a/tests/f_multithread/script b/tests/f_multithread/script
index 0fe96cd0..83cd0f03 100644
--- a/tests/f_multithread/script
+++ b/tests/f_multithread/script
@@ -1,4 +1,4 @@
-FSCK_OPT="-fy -m"
+FSCK_OPT="-fy -m1"
 SECOND_FSCK_OPT=-yf
 
 . $cmd_dir/run_e2fsck
diff --git a/tests/f_multithread_completion/script b/tests/f_multithread_completion/script
index bf23cd61..0ec13816 100644
--- a/tests/f_multithread_completion/script
+++ b/tests/f_multithread_completion/script
@@ -1,4 +1,4 @@
-FSCK_OPT="-fy -m -C 1"
+FSCK_OPT="-fy -m1 -C 1"
 SECOND_FSCK_OPT=-yf
 
 . $cmd_dir/run_e2fsck
diff --git a/tests/f_multithread_logfile/script b/tests/f_multithread_logfile/script
index 4f9ca6f8..dbb65319 100644
--- a/tests/f_multithread_logfile/script
+++ b/tests/f_multithread_logfile/script
@@ -1,5 +1,5 @@
 LOG_FNAME="f_multithread_logfile_xxx"
-FSCK_OPT="-fy -m -y -E log_filename=$LOG_FNAME"
+FSCK_OPT="-fy -m1 -y -E log_filename=$LOG_FNAME"
 SKIP_VERIFY="true"
 ONE_PASS_ONLY="true"
 SKIP_CLEANUP="true"
diff --git a/tests/f_multithread_no/script b/tests/f_multithread_no/script
index b93deb3a..db791e11 100644
--- a/tests/f_multithread_no/script
+++ b/tests/f_multithread_no/script
@@ -1,4 +1,4 @@
-FSCK_OPT="-fn -m"
+FSCK_OPT="-fn -m1"
 SECOND_FSCK_OPT=-yf
 
 . $cmd_dir/run_e2fsck
diff --git a/tests/f_multithread_preen/script b/tests/f_multithread_preen/script
index ecb79cd6..8965f4a7 100644
--- a/tests/f_multithread_preen/script
+++ b/tests/f_multithread_preen/script
@@ -1,4 +1,4 @@
-FSCK_OPT="-fp -m"
+FSCK_OPT="-fp -m1"
 SECOND_FSCK_OPT=-yf
 
 . $cmd_dir/run_e2fsck
diff --git a/tests/f_multithread_yes/script b/tests/f_multithread_yes/script
index 38891f6a..8b4aa9b8 100644
--- a/tests/f_multithread_yes/script
+++ b/tests/f_multithread_yes/script
@@ -1,4 +1,4 @@
-FSCK_OPT="-f -m"
+FSCK_OPT="-f -m1"
 SECOND_FSCK_OPT=-yf
 
 . $cmd_dir/run_e2fsck
