Re: [PATCH] mmu notifiers #v8

Nick Piggin
2008-03-03T13:10:31Z

On Mon, Mar 03, 2008 at 01:51:53PM +0100, Andrea Arcangeli wrote:
> On Mon, Mar 03, 2008 at 04:29:34AM +0100, Nick Piggin wrote:
> > to something I prefer. Others may not, but I'll post them for debate
> > anyway.
>
> Sure, thanks!
>
> > > I didn't drop invalidate_page, because invalidate_range_begin/end
> > > would be slower for usages like KVM/GRU (we don't need a begin/end
> > > there because where invalidate_page is called, the VM holds a
> > > reference on the page). do_wp_page should also use invalidate_page
> > > since it can free the page after dropping the PT lock without losing
> > > any performance (that's not true for the places where invalidate_range
> > > is called).
> >
> > I'm still not completely happy with this. I had a very quick look
> > at the GRU driver, but I don't see why it can't be implemented
> > more like the regular TLB model, and have TLB insertions depend on
> > the linux pte, and do invalidates _after_ restricting permissions
> > to the pte.
> >
> > Ie. I'd still like to get rid of invalidate_range_begin, and get
> > rid of invalidate calls from places where permissions are relaxed.
>
> _begin exists because by the time _end is called, the VM already
> dropped the reference on the page. This way we can do a single
> invalidate no matter how large the range is. I don't see ways to
> remove _begin while still invoking _end a single time for the whole
> range.

Is this just a GRU problem? Can't we just require them to take a ref
on the page (IIRC Jack said GRU could be changed to more like a TLB
model).