> However, for things like executables and libraries, that are mapped
> read-only, we'd only have to zero the page if some other process had
> opened the file in write mode since the last time we zero'd it, which
> would probably drop the overhead for those to ~0.

Not only that, but, you don't zero the page; you zero the partial pages
which are after EOF but which are mapped anyway because there is a
partial page before EOF.  For normal executables and shared liberaries,
that means you don't zero anything, since they are all page-aligned
maps of page-aligned regions which are wholly backed by the files.  The
only overhead in these cases is testing to see whether you need to zero
anything.

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