Re: [PATCH 1/3] mmu_notifier: Add mmu_notifier_invalidate_range()

From: Jerome Glisse
Date: Fri Jul 25 2014 - 16:43:38 EST

On Fri, Jul 25, 2014 at 01:16:39PM -0700, Jesse Barnes wrote:
> On Thu, 24 Jul 2014 16:35:39 +0200
> Joerg Roedel <joro@xxxxxxxxxx> wrote:
> > From: Joerg Roedel <jroedel@xxxxxxx>
> >
> > This notifier closes an important gap with the current
> > invalidate_range_start()/end() notifiers. The _start() part
> > is called when all pages are still mapped while the _end()
> > notifier is called when all pages are potentially unmapped
> > and already freed.
> >
> > This does not allow to manage external (non-CPU) hardware
> > TLBs with MMU-notifiers because there is no way to prevent
> > that hardware will establish new TLB entries between the
> > calls of these two functions. But this is a requirement to
> > the subsytem that implements these existing notifiers.
> >
> > To allow managing external TLBs the MMU-notifiers need to
> > catch the moment when pages are unmapped but not yet freed.
> > This new notifier catches that moment and notifies the
> > interested subsytem when pages that were unmapped are about
> > to be freed. The new notifier will only be called between
> > invalidate_range_start()/end().
> So if we were actually sharing page tables, we should be able to make
> start/end no-ops and just use this new callback, assuming we didn't
> need to do any other serialization or debug stuff, right?
> Seems like a good addition, and saves us a bunch of trouble...

Pondering on that i think there is a missing call to mmu_notifier_invalidate_range
inside move_huge_pmd which is call by move_page_tables.

But otherwise yes, you should not need to register range_start/end() callback. It
should be enought to only register the invalidate_range callback.

Note that on my side i will remain an user of range_start/end() but other listener
like kvm or xen or sgi might want to revisit there code with this new callback.


> Thanks,
> --
> Jesse Barnes, Intel Open Source Technology Center
> --
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