On Mon, Jun 21, 2021 at 08:26:56AM +0200, Christoph Hellwig wrote:

Split get_fs_names into one function that splits up the command line
argument, and one that gets the list of all registered file systems.

+static void __init get_all_fs_names(char *page)
+{
+	int len = get_filesystem_list(page);
+	char *s = page, *p, *next;
+
+	page[len] = '\0';
+	for (p = page - 1; p; p = next) {
+		next = strchr(++p, '\n');
+		if (*p++ != '\t')
+			continue;
+		while ((*s++ = *p++) != '\n')
+			;
+		s[-1] = '\0';
 	}
+
 	*s = '\0';
 }

TBH, I would rather take that one into fs/filesystems.c.  Rationale:
get_filesystem_list(), for all its resemblance to /proc/filesystems
contents, is used only by init/*.c and it's not a big deal to make
it

int __init get_filesystem_list(char *buf, bool is_dev)
{
	int f = is_dev ? FS_REQUIRES_DEV : 0;
        int left = PAGE_SIZE, count = 0;
        struct file_system_type *p;

        read_lock(&file_systems_lock);
	for (p = file_systems; p; p = p->next) {
		if ((p->fs_flags & FS_REQUIRES_DEV) == f) {
			size_t len = strlen(p->name) + 1;
			if (len > left)
				break;
			memcpy(buf, p->name, len);
			buf += len;
			left -= len;
			count++;
		}
	}
        read_unlock(&file_systems_lock);
	return count;
}

Generates NUL-separated list, returns the number of list elements,
the second argument is "what kind do you want"...