Function pread_at_least 

pub fn pread_at_least(
    file: &File,
    offset: u64,
    dst: &mut [u8],
    need: usize,
) -> Result<()>

pread into dst until at least need bytes have landed (need <= dst.len()): the direct miss path reads 512 B windows whose tail may run past the end of the file, where only the tensor’s own bytes are owed.