[ofa-general] Re: [patch] my mmu notifiers

[Nick Piggin]

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.