linux-kernel - Re: [patch] my mmu notifiers

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Date:	Thu, 21 Feb 2008 05:42:56 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Jack Steiner <steiner@....com>
Cc:	Andrea Arcangeli <andrea@...ranet.com>, akpm@...ux-foundation.org,
	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Christoph Lameter <clameter@....com>
Subject: Re: [patch] my mmu notifiers

On Tue, Feb 19, 2008 at 05:40:50PM -0600, Jack Steiner wrote:
> On Wed, Feb 20, 2008 at 12:11:57AM +0100, Nick Piggin wrote:
> > On Tue, Feb 19, 2008 at 02:58:51PM +0100, Andrea Arcangeli wrote:
> > > On Tue, Feb 19, 2008 at 09:43:57AM +0100, Nick Piggin wrote:
> > > > anything when changing the pte to be _more_ permissive, and I don't
> > > 
> > > Note that in my patch the invalidate_pages in mprotect can be
> > > trivially switched to a mprotect_pages with proper params. This will
> > > prevent page faults completely in the secondary MMU (there will only
> > > be tlb misses after the tlb flush just like for the core linux pte),
> > > and it'll allow all the secondary MMU pte blocks (512/1024 at time
> > > with my PT lock design) to be updated to have proper permissions
> > > matching the core linux pte.
> > 
> > Sorry, I realise I still didn't get this through my head yet (and also
> > have not seen your patch recently). So I don't know exactly what you
> > are doing...
> > 
> > But why does _anybody_ (why does Christoph's patches) need to invalidate
> > when they are going to be more permissive? This should be done lazily by
> > the driver, I would have thought.
> 
> 
> Agree. Although for most real applications, the performance difference
> is probably negligible.

But importantly, doing it that way means you share test coverage with
the CPU TLB flushing code, and you don't introduce a new concept to the
VM.

So, it _has_ to be lazy flushing, IMO (as there doesn't seem to be a
good reason otherwise). mprotect shouldn't really be a special case,
because it still has to flush the CPU tlbs as well when restricting
access.
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