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Re: mmap modified data not to be swapped out but saved?

Ben Dougall asked: 
with mmap once you've read in some of a file and made some
modifications is there anyway to avoid those modifications being
swapped out to the backing store? what would be preferable is in the
event of the system needing more memory, rather than swapping out those
modifications, save them directly to the original file -- is that what
happens now, by default maybe, or is there anyway of doing that?

While I haven't examined Darwin's code lately, I am pretty familiar with
the Mach's VM code from years past, and my memory says that what you
want to have happen is supported. (At pageout, "dirty" pages are
written back to the file backing the mapped region, which is a file-backed
Memory Object). Revisiting the mmap man page reminds me that this
might only be the case if the region is mmapped including the
MAP_SHARED flag.

A simple experiment should validate this hypothesis: try it, and drive
some paging!

Have fun!
Dylan McNamee

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