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In the general case with an overlay, all mmap calls must go through the
overlay, because in the event of a copy-up, the overlay needs to invalidate any
previously-created mappings.
If there if no lower file, however, there will never be a copy-up, so the
overlay can delegate directly to the upper file in that case.
This also allows us to correctly mmap /dev/zero when it is in an overlay. This
file has special semantics which the overlay does not know about. In
particular, it does not implement Mappable(), which (in the general case) the
overlay uses to detect if a file is mappable or not.
PiperOrigin-RevId: 205306743
Change-Id: I92331649aa648340ef6e65411c2b42c12fa69631
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