Unmapping and remapping is dangerous since another thread could touch
the shadow memory while it is unmapped. But there is really no need to
unmap anyway, since mmap(MAP_FIXED) will happily clobber the existing
mapping with zeroes. This is thread-safe since the mmap() is done under
the same kernel lock as page faults are done.
Reviewed By: vitalybuka
Differential Revision: https://reviews.llvm.org/D85947
Matt Morehouse