On 14 Jul 1999, Chris G. Demetriou wrote:

> Doug Rabson <dfr@nlsystems.com> writes:
> > Overcommit can be used for many reasons. I use it to reserve a large
> > linear address space to mmap alpha i/o spaces [...]
> 
> Overcommit can be used for many reasons, but unless you've
> misdescribed what you're doing, _that's not one of them_.
> 
> The mapped I/O pages need no backing store to be allocated for them by
> the VM system.  They're backed by hardware.
> 
> And if you have 'placeholder' pages (I note that you didn't say you
> mmap all of alpha i/o space, just reserve a large linear address space
> in which to mmap it), then it should be possible to map them in such a
> way (e.g. read-only ZFOD) in which they wouldn't count against backing
> store requirements, either.

I certainly don't need or want backing store for these pages. The original
reserved region is never touched without first mapping device pages onto
it.

--
Doug Rabson				Mail:  dfr@nlsystems.com
Nonlinear Systems Ltd.			Phone: +44 181 442 9037