Re: storage-class memory (was: Re: state of XIP?)

[David Holland <dholland-tech%netbsd.org@localhost>]

On Fri, Oct 18, 2013 at 09:15:33AM -0700, Matt Thomas wrote:
 > >> The only problem is marking data as copy-on-write
 > >> but again these pages aren't managed so the current COW code won't
 > >> be happy.
 > > 
 > > We shouldn't have to care about that unless we want to move to
 > > MAP_COPY from MAP_PRIVATE.
 > 
 > Huh?  I'm was talking about an executable's .data section.
 > Since we are talking about execute-in-place.

If you support MAP_COPY, then you have to support copying-on-write on
the filesystem's access path to file pages. If the file page is
something physically significant, this becomes problematic.

If you don't support MAP_COPY but only MAP_PRIVATE, then all that
matters is the mapping, so all you have to do is enter it into the
pmap as readonly when you fault it in, and it shouldn't matter what
kind of page it is underneath.

But, as I've said, while I've written more than one VM system I don't
know that much about UVM specifically, so maybe there are issues I'm
not thinking of.

-- 
David A. Holland
dholland%netbsd.org@localhost