On Thu, May 18, 2000 at 03:52:30PM +0200, Erik Corry wrote:
> > The pages you have not written to all point to the
> > zero-filled page. If you set up the mapping you'll
> > have 256k mappings to that one page :)
>
> Hmm. Quite misleading but at least internally consistent in some
> way. Normally I wouldn't expect the RSS to count a given physical
> page more than once.
That's the sematics of RSS. I guess it's that way because it's older
than shared pages in UNIX.
> > > Do I need more real RAM to fork if I have a large mmap like this?
> >
> > No. You will only need more RAM (or swap) if you
> > write to a lot of the pages, otherwise they'll
> > keep pointing (COW) to the zero-filled page.
>
> But I do need the 4k-page per 4Mbyte mapping to be duplicated, right?
> Or is that copy-on-write too?
No, the 4kb page is globally shared between all mappings in all processes.
It's a statically allocated page that is allocated on startup. On
certain MIPS CPUs we even allocated a set of 8 pages.
Ralf
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