If I mmap an area of anonymous memory and set the protection bits to
write only will it be associated with any physical memory?  I assume it
isn't filled until there is a page fault, which shouldn't fill it anyway
because it's write only.  Are these assumptions correct?  I'd like to put
a cap on a second stack so that it doesn't overflow onto other memory on
the heap, but I don't want the cap to waste memory.

This leads me to a second question.. Where are the stacks allocated for
the upcalls in the scheduler activations?  Are these stacks growable? And
if not, are they capped?

I apologise if this isn't the appropriate place to ask, but I don't know
anyone else who might know the answer.

Thanks,
Jeff