Revert r329254. The effect of r329254 was to perform COW faults as two page faults, not one. The first fault updated the mapped physical address, but mapped the new physical page with read-only access, and the second fault added write access. Over the past few weeks, we have reviewed and updated the various pmap implementation so that this extra fault wasn't necessary.
I did a poudriere run to build my usual set of ports on r336438 with this patch and didn't observe any unexpected fallout. In particular lang/ghc and lang/go both built successfully, which almost always failed prior to the original fix. This was on amd64, so I can't say anything about other arches.