double allocations:

[bill]

I've noticed that an allocation request for a large chunk of memory 
(128GB) results in two calls to pages_map() (in src/chunk_mmap.c), 
consuming 2x the VM I requested.  In a 64 bit world this is not a big 
problem, but I've recoded pages_map() to force allocation from an mmap'd 
ssd (instead of swap anonymous mmap), and it's forcing me to run out of 
backing store.  The issue I would like to understand is why pages_map() 
is called twice with separate requests for the single 128GB jemalloc() 
that I'm doing in my application.  The first allocation is followed by a 
call to pages_unmap(), but with an unmap size of 0 bytes, leaving it 
fully mapped, while the second allocation (which is slightly larger than 
128GB) is trimmed to exactly 128GB by 2 subsequent pages_unmap() calls.  
This behavior seems very strange to me, and any explanation would be 
appreciated.
Bill