Constant fs_verity::MAX_LEVELS

pub const MAX_LEVELS: usize = 8;

Linux has a hardcoded limit of 8, see FS_VERITY_MAX_LEVELS in /fs/verity/fsverity_private.h in the Linux source. In reality you are not likely to hit this limit ever, e.g. with SHA256 you’d need more than usize::MAX input bytes.