kernel code reading: netlink message type
kernel code reading: net_device and netdev_queue
kernel code reading: kernfs file add

kernfs_node could represent a file or a dir, all link to parent's dir rb tree.
userspace code test: simple ls
kernel code reading: read syscall

fd -> file -> file->f_op->read
kernel code reading: syscall for cd
userspace code test: test on fd returned by open
kernel code test: radix tree example
kernel code test: traverse a list tree iterately
kernel code reading: how a file is opened
kernel code reading: kernfs_nod <-> dentry