On Mon, Aug 14, 2017 at 11:12 PM, Dan Williams [off-list ref] wrote:

The mmap syscall suffers from the ABI anti-pattern of not validating
unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a
mechanism to define new behavior that is known to fail on older kernels
without the feature. Use the fact that specifying MAP_SHARED and
MAP_PRIVATE at the same time is invalid as a cute hack to allow a new
set of validated flags to be introduced.

While this is cute, is it actually better than a new syscall?