I'm considering to use an encrypted file system (dm-crypt or gocryptfs) in a linux system hosted by Linode. Can the administrators of Linode access the mounted (i.e., decrypted) contents? I'm not sure which technology they use to provide a virtual system. But anyway I've installed Arch linux and have a full control on the system. I think they cannot modify the kernel and system softwares provided by Arch. So I think that as long as they don't know my root password, it's very hard to read the mounted contents. Is that true?
No, this would likely not be a safe assumption. Generally speaking, if you are running software on systems outside your own physical control, you must place near full or at least a significant level of trust in whoever does have physical control of the system.
Something that would likely be trivial for a cloud provider is to take a full dump of your virtual system's memory (RAM). Since your VM will be in a decrypted state while it is running, there will likely be decryption keys or other sensitive data that can be extracted from the RAM dump. This may be enough for Linode to decrypt the full contents of the filesystem.
See this related question for further reading.