On Wed, Mar 10, 2021, Jason Gunthorpe wrote:
> On Wed, Mar 10, 2021 at 05:20:01PM -0800, Sean Christopherson wrote:
>
> > > Which I believe is fatal to kvm? These notifiers certainly do not only
> > > happen at process exit.
> >
> > My point about the process dying is that the existing bug that causes
> > mmu_notifier_count to become imbalanced is benign only because the process is
> > being killed, and thus KVM will stop running its vCPUs.
>
> Are you saying we only call non-blocking invalidate during a process
> exit event??
Yes? __oom_reap_task_mm() is the only user of _nonblock(), if that's what you're
asking.
> > > So, both of the remaining _end users become corrupted with this patch!
> >
> > I don't follow. mn_hlist_invalidate_range_start() iterates over all
> > notifiers, even if a notifier earlier in the chain failed. How will
> > KVM become imbalanced?
>
> Er, ok, that got left in a weird way. There is another "bug" where end
> is not supposed to be called if the start failed.
Ha, the best kind of feature. :-)
> > The existing _end users never fail their _start. If KVM started failing its
> > start, then yes, it could get corrupted.
>
> Well, maybe that is the way out of this now. If we don't permit a
> start to fail if there is an end then we have no problem to unwind it
> as we can continue to call everything. This can't be backported too
> far though, the itree notifier conversions are what made the WARN_ON
> safe today.
>
> Something very approximately like this is closer to my preference:
Makes sense. I don't have a strong preference, I'll give this a spin tomorrow.
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 61ee40ed804ee5..6d5cd20f81dadc 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -501,10 +501,25 @@ static int mn_hlist_invalidate_range_start(
> "");
> WARN_ON(mmu_notifier_range_blockable(range) ||
> _ret != -EAGAIN);
> + /*
> + * We call all the notifiers on any EAGAIN,
> + * there is no way for a notifier to know if
> + * its start method failed, thus a start that
> + * does EAGAIN can't also do end.
> + */
> + WARN_ON(ops->invalidate_range_end);
> ret = _ret;
> }
> }
> }
> +
> + if (ret) {
> + /* Must be non-blocking to get here*/
> + hlist_for_each_entry_rcu (subscription, &subscriptions->list,
> + hlist, srcu_read_lock_held(&srcu))
> + subscription->ops->invalidate_range_end(subscription,
> + range);
> + }
> srcu_read_unlock(&srcu, id);
>
> return ret;
