[Bug 240061] MADV_FREE rewinds time to before fork()

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240061

--- Comment #6 from Mark Johnston <markj at FreeBSD.org> ---
One possible alternative is to convert MADV_FREE to MADV_DONTNEED if the object
has a backing object.  pmap_advise() needs to be updated to preserve the
modified bits, but this should be fine since vm_page_advise(MADV_FREE) calls
vm_page_undirty() anyway.

Some basic testing on my workstation showed that virtually all calls to
vm_object_madvise(MADV_FREE) are acting on an object with no backing object. 
Obviously this may not be true in general.

# dtrace -n 'fbt::vm_object_madvise:entry /args[0] && args[3] ==
5/{@[args[0]->backing_object == NULL] = count();}'
^C
        0               13
        1            17504

Untested patch here: https://people.freebsd.org/~markj/patches/madvise_cow.diff

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