Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

Nick Piggin
2007-03-07T11:00:46Z

On Wed, Mar 07, 2007 at 11:47:42AM +0100, Peter Zijlstra wrote:
> On Wed, 2007-03-07 at 11:38 +0100, Nick Piggin wrote:
>
> > > > There are real users who want these fast, though.
> > >
> > > Yeah, why don't we have a tree per nonlinear vma to find these pages?
> > >
> > > wli mentions shadow page tables..
> >
> > We could do something more efficient, but I thought that half the point
> > was that they didn't carry any of this extra memory, and they could be
> > really fast to set up at the expense of efficiency elsewhere.
>
> I'm failing to understand this :-(
>
> That extra memory, and apparently they don't want the inefficiency
> either.

Sorry, I didn't understand your misunderstandings ;)

>
> > I don't see it being a big deal. I doubt anybody is writing out huge
> > amounts of data via nonlinear mappings.
>
> Well, now they don't, but it could be done or even exploited as a DoS.

But so could nonlinear page reclaim. I think we need to restrict nonlinear
mappings to root if we're worried about that.