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

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Message-ID: <20070307110035.GE5555@wotan.suse.de>
Date:	Wed, 7 Mar 2007 12:00:36 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	mingo@...e.hu, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org
Subject: Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

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.
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