diff --git a/Documentation/mkfs.btrfs.rst b/Documentation/mkfs.btrfs.rst
index 0e9e84ad..629231a2 100644
--- a/Documentation/mkfs.btrfs.rst
+++ b/Documentation/mkfs.btrfs.rst
@@ -155,11 +155,15 @@ OPTIONS
                 contain the files from *rootdir*. Since version 4.14.1 the filesystem size is
                 not minimized. Please see option *--shrink* if you need that functionality.
 
--u|--subvol <subdir>
+-u|--subvol <subdir>:<flags>
         Specify that *subdir* is to be created as a subvolume rather than a regular
         directory.  The option *--rootdir* must also be specified, and *subdir* must be an
         existing subdirectory within it.  This option can be specified multiple times.
 
+        *flags* is an optional comma-separated list of modifiers. Valid choices are:
+
+        * *default*: create as default subvolume (this can only be specified once)
+
 --shrink
         Shrink the filesystem to its minimal size, only works with *--rootdir* option.
 
diff --git a/mkfs/main.c b/mkfs/main.c
index 57d9fcc7..b0d4fd0b 100644
--- a/mkfs/main.c
+++ b/mkfs/main.c
@@ -440,7 +440,7 @@ static const char * const mkfs_usage[] = {
 	"Creation:",
 	OPTLINE("-b|--byte-count SIZE", "set size of each device to SIZE (filesystem size is sum of all device sizes)"),
 	OPTLINE("-r|--rootdir DIR", "copy files from DIR to the image root directory"),
-	OPTLINE("-u|--subvol SUBDIR", "create SUBDIR as subvolume rather than normal directory, can be specified multiple times"),
+	OPTLINE("-u|--subvol SUBDIR:FLAGS", "create SUBDIR as subvolume rather than normal directory, can be specified multiple times"),
 	OPTLINE("--shrink", "(with --rootdir) shrink the filled filesystem to minimal size"),
 	OPTLINE("-K|--nodiscard", "do not perform whole device TRIM"),
 	OPTLINE("-f|--force", "force overwrite of existing filesystem"),
@@ -1015,6 +1015,46 @@ static void *prepare_one_device(void *ctx)
 	return NULL;
 }
 
+static int parse_subvol_flags(struct rootdir_subvol *subvol, const char *flags)
+{
+	char *buf, *orig_buf;
+	int ret;
+
+	buf = orig_buf = strdup(flags);
+
+	if (!buf) {
+		error_msg(ERROR_MSG_MEMORY, NULL);
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	while (true) {
+		char *comma = strstr(buf, ",");
+
+		if (comma)
+			*comma = 0;
+
+		if (!strcmp(buf, "default")) {
+			subvol->is_default = true;
+		} else if (buf[0] != 0) {
+			error("unrecognized subvol flag \"%s\"", buf);
+			ret = 1;
+			goto out;
+		}
+
+		if (comma)
+			buf = comma + 1;
+		else
+			break;
+	}
+
+	ret = 0;
+
+out:
+	free(orig_buf);
+	return ret;
+}
+
 int BOX_MAIN(mkfs)(int argc, char **argv)
 {
 	char *file;
@@ -1058,6 +1098,7 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 	char *source_dir = NULL;
 	size_t source_dir_len = 0;
 	struct rootdir_subvol *rds;
+	bool has_default_subvol = false;
 	LIST_HEAD(subvols);
 
 	cpu_detect_flags();
@@ -1215,16 +1256,49 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 				break;
 			case 'u': {
 				struct rootdir_subvol *subvol;
+				char *colon;
 
-				subvol = malloc(sizeof(struct rootdir_subvol));
+				subvol = calloc(1, sizeof(struct rootdir_subvol));
 				if (!subvol) {
 					error_msg(ERROR_MSG_MEMORY, NULL);
 					ret = 1;
 					goto error;
 				}
 
-				subvol->dir = strdup(optarg);
-				subvol->full_path = NULL;
+				colon = strstr(optarg, ":");
+
+				if (colon) {
+					/* Make sure we choose the last colon in
+					 * optarg, in case the subvol name
+					 * itself contains a colon.  */
+					do {
+						char *colon2;
+
+						colon2 = strstr(colon + 1, ":");
+
+						if (colon2)
+							colon = colon2;
+						else
+							break;
+					} while (true);
+
+					subvol->dir = strndup(optarg, colon - optarg);
+					if (parse_subvol_flags(subvol, colon + 1)) {
+						ret = 1;
+						goto error;
+					}
+				} else {
+					subvol->dir = strdup(optarg);
+				}
+
+				if (subvol->is_default) {
+					if (has_default_subvol) {
+						error("subvol default flag can only be specified once");
+						ret = 1;
+						goto error;
+					}
+					has_default_subvol = true;
+				}
 
 				list_add_tail(&subvol->list, &subvols);
 				break;
diff --git a/mkfs/rootdir.c b/mkfs/rootdir.c
index 3cc94316..c31651d0 100644
--- a/mkfs/rootdir.c
+++ b/mkfs/rootdir.c
@@ -99,6 +99,7 @@ static u64 g_hardlink_count;
 static struct btrfs_trans_handle *g_trans = NULL;
 static struct list_head *g_subvols;
 static u64 next_subvol_id = BTRFS_FIRST_FREE_OBJECTID;
+static u64 default_subvol_id;
 
 static inline struct inode_entry *rootdir_path_last(struct rootdir_path *path)
 {
@@ -436,6 +437,9 @@ static int ftw_add_subvol(const char *full_path, const struct stat *st,
 		return ret;
 	}
 
+	if (subvol->is_default)
+		default_subvol_id = subvol_id;
+
 	key.objectid = subvol_id;
 	key.type = BTRFS_ROOT_ITEM_KEY;
 	key.offset = (u64)-1;
@@ -701,6 +705,47 @@ static int ftw_add_inode(const char *full_path, const struct stat *st,
 	return 0;
 };
 
+static int set_default_subvolume(struct btrfs_trans_handle *trans)
+{
+	struct btrfs_path path = { 0 };
+	struct btrfs_dir_item *di;
+	struct btrfs_key location;
+	struct extent_buffer *leaf;
+	struct btrfs_disk_key disk_key;
+	u64 features;
+
+	di = btrfs_lookup_dir_item(trans, trans->fs_info->tree_root, &path,
+				   btrfs_super_root_dir(trans->fs_info->super_copy),
+				   "default", 7, 1);
+	if (IS_ERR_OR_NULL(di)) {
+		btrfs_release_path(&path);
+
+		if (di)
+			return PTR_ERR(di);
+		else
+			return -ENOENT;
+	}
+
+	leaf = path.nodes[0];
+
+	location.objectid = default_subvol_id;
+	location.type = BTRFS_ROOT_ITEM_KEY;
+	location.offset = 0;
+
+	btrfs_cpu_key_to_disk(&disk_key, &location);
+	btrfs_set_dir_item_key(leaf, di, &disk_key);
+
+	btrfs_mark_buffer_dirty(leaf);
+
+	btrfs_release_path(&path);
+
+	features = btrfs_super_incompat_flags(trans->fs_info->super_copy);
+	features |= BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL;
+	btrfs_set_super_incompat_flags(trans->fs_info->super_copy, features);
+
+	return 0;
+}
+
 int btrfs_mkfs_fill_dir(struct btrfs_trans_handle *trans, const char *source_dir,
 			struct btrfs_root *root, struct list_head *subvols)
 {
@@ -732,6 +777,14 @@ int btrfs_mkfs_fill_dir(struct btrfs_trans_handle *trans, const char *source_dir
 	while (current_path.level > 0)
 		rootdir_path_pop(&current_path);
 
+	if (default_subvol_id != 0) {
+		ret = set_default_subvolume(trans);
+		if (ret < 0) {
+			error("error setting default subvolume: %d", ret);
+			return ret;
+		}
+	}
+
 	return 0;
 }
 
diff --git a/mkfs/rootdir.h b/mkfs/rootdir.h
index 128e9e09..871889d9 100644
--- a/mkfs/rootdir.h
+++ b/mkfs/rootdir.h
@@ -32,6 +32,7 @@ struct rootdir_subvol {
 	struct list_head list;
 	char *dir;
 	char *full_path;
+	bool is_default;
 };
 
 int btrfs_mkfs_fill_dir(struct btrfs_trans_handle *trans, const char *source_dir,