SHM problem with exactly 16GB RAM

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've found that Apache does not start if my netbsd-7 amd64 domU has 16GB 
of RAM. 15.9GB and 16.1GB are OK. It looks to be related to shared memory.

Errors are along the lines of:

Invalid argument: mod_fcgid: Can't create shared memory for size 1200712 bytes
Or:
[:emerg] [pid 1868] AH02818: MPM run failed, exiting

I also see the following even with a working system if the memory is large 
(e.g. 15.9GB):

balloon0: current reservation: 4168192 pages => target: 4168090 pages
balloon0: increase reservation incomplete: was 410, returned 409
balloon0: WARNING: balloon could not reach target 4168090 (current 4167680)

This reminds me of a problem many years ago with an i386 domU. If the RAM 
was set to exactly 4GB it allocated 0GB to the VM which failed to start.

--
Stephen