Subject: Re: Replacement for grep(1) (part 2)
To: Chris G. Demetriou <>
From: Doug Rabson <>
List: tech-userlevel
Date: 07/14/1999 20:29:44
On 14 Jul 1999, Chris G. Demetriou wrote:

> Doug Rabson <> 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

Doug Rabson				Mail:
Nonlinear Systems Ltd.			Phone: +44 181 442 9037