What does align_log2 in alloc_contiguous means?

[stevefan1999-personal]

I'm trying to allocate a 2MiB huge frame like this:

unsafe impl<T: PageSize> FrameAllocator<T> for GlobalFrameAllocator {
    fn allocate_frame(&mut self) -> Option<PhysFrame<T>> {
        unsafe {
            if let Some(addr) = BITMAP.alloc_contiguous(T::SIZE as usize / Size4KiB::SIZE as usize, 1) {
                let addr = PhysAddr::new(addr as u64 * Size4KiB::SIZE);
                PhysFrame::from_start_address(addr).ok()
            } else {
                None
            }
        }
    }
}

impl<T: PageSize> FrameDeallocator<T> for GlobalFrameAllocator {
    unsafe fn deallocate_frame(&mut self, frame: PhysFrame<T>) {
        let addr = frame.start_address().as_u64() as usize;
        BITMAP.dealloc(addr);
    }
}

I'm very confused on what alloc_contiguous does