DIMMs are not necessarily mapped continuously. Often they are mapped
"around" holes. Or some parts of the DIMM disappear magically for
some hidden purpose like SMM. The actual DIMM sizes don't matter, but what
matters is what memory map the kernel sees after all translation
done in other layers. And that end result is not necessarily
aligned to 2MB.
In that case pretty much all 64bit kernels will overmap slightly,
32bit kernels sometimes (e.g. under the right PAGE_OFFSET split
or when the <4GB hole is very large)
-Andi
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