This has been fixed in FreeBSD:
http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/48291
http://marc.theaimsgroup.com/?l=freebsd-cvs-all&m=106741759426858&w=2

The fix is (basically) to delay sending the close SMB to the server
until VOP_INACTIVE - the reference counting that smbfs does in
VOP_OPEN and VOP_CLOSE does not account for mmap()'d regions.
It's kinda hard to service a page fault reading from a file that
has already been closed on the server :-)

Also note that even though I had no real plans to merge it into
FreeBSD RELENG_4, I ended up doing it due to demand. Some of this
patchset might be useful to NetBSD (it includes the mmap fix):
http://marc.theaimsgroup.com/?l=freebsd-cvs-all&m=107370830511420&w=2


Tim