Since the contents are in-memory, you could imagine a straightforward implementation where the directory hierarchy in the tmpfs is stored in a pointer-based in-

Since the contents are in-memory, you could imagine a straightforward implementation where the directory hierarchy in the tmpfs is stored in a pointer-based in-memory tree. A file handle just needs a pointer to where in memory the contents are stored, and the inode numbers are computed on the side because it's part of the interface. You don't want to use the pointers as inode numbers directly, since for security reasons the kernel doesn't want to export valid kernel pointers to user space.