Why ioremap allocated areas have such large aligment?

Question

I have a code that fails calling ioremap() for 4M region. Trying to debug the reason, I've found out that if you call ioremap it will try to allocate continuous addresses with a very large alignment (depending on the size of the area you want to allocate). The code that computes this alignment is in __get_vm_area_node() function (mm/vmalloc.c) and it looks like this:

if (flags & VM_IOREMAP) {
    int bit = fls(size);

    if (bit > IOREMAP_MAX_ORDER)
        bit = IOREMAP_MAX_ORDER;
    else if (bit < PAGE_SHIFT)
        bit = PAGE_SHIFT;

    align = 1ul << bit;
}

On ARM, IOREMAP_MAX_ORDER is defined as 23. This means that in my case, ioremap needs not only 4M of continues addressing in vmalloc area but it also has to be aligned to 4M.

I wasn't able to find any information on why this alignment is needed. I even tried using git blame to see the commit that introduces this change but it seems the code is older than git history so I couldn't find anything.