Re: [PATCH] kvm/x86: Handle async PF in RCU read-side critical sections

[Paul E. McKenney]

On Mon, Oct 02, 2017 at 02:45:34PM +0200, Paolo Bonzini wrote:
> On 30/09/2017 19:15, Paul E. McKenney wrote:
> > On Sat, Sep 30, 2017 at 07:41:56AM +0800, Boqun Feng wrote:
> >> On Fri, Sep 29, 2017 at 04:43:39PM +0000, Paul E. McKenney wrote:
> >>> Not to be repetitive, but if the schedule() is on the guest, this change
> >>> really does silently break up an RCU read-side critical section on
> >>> guests built with PREEMPT=n.  (Yes, they were already being broken,
> >>> but it would be good to avoid this breakage in PREEMPT=n as well as
> >>> in PREEMPT=y.)
> 
> Yes, you're right.  It's pretty surprising that it's never been reported.

It would look like random memory corruption in the guest, so it might
well have been encountered.  Though you have to get a page fault in
just the wrong place and an update has to happen just at that time, so
perhaps low probability.

Still, good to fix.

> >> Then probably adding !IS_ENABLED(CONFIG_PREEMPT) as one of the reason we
> >> choose the halt path? Like:
> >>
> >> 	n.halted = is_idle_task(current) || preempt_count() > 1 ||
> >> 		   !IS_ENABLED(CONFIG_PREEMPT) || rcu_preempt_depth();
> >>
> >>
> >> But I think async PF could also happen while a user program is running?
> >> Then maybe add a second parameter @user for kvm_async_pf_task_wait(),
> >> like:
> >>
> >> 	kvm_async_pf_task_wait((u32)read_cr2(), user_mode(regs));
> >>
> >> and the halt condition becomes:
> >>
> >> 	n.halted = is_idle_task(current) || preempt_count() > 1 ||
> >> 		   (!IS_ENABLED(CONFIG_PREEMPT) && !user) || rcu_preempt_depth();
> >>
> >> Thoughts?
> > 
> > This looks to me like it would cover it.  If !PREEMPT interrupt from
> > kernel, we halt, which would prevent the sleep.
> > 
> > I take it that we get unhalted when the host gets things patched up?
> 
> Yes.  You get another page fault (this time it's a "page ready" page
> fault rather than a "page not present" one), which has the side
> effecting of ending the halt.

Got it, thank you!

							Thanx, Paul

> Paolo
> 
> >> A side thing is being broken already for PREEMPT=n means we maybe fail
> >> to detect this in rcutorture? Then should we add a config with
> >> KVM_GUEST=y and try to run some memory consuming things(e.g. stress
> >> --vm) in the rcutorture kvm script simultaneously? Paolo, do you have
> >> any test workload that could trigger async PF quickly?
> > 
> > I do not believe that have seen this in rcutorture, but I always run in
> > a guest OS on a large-memory system (well, by my old-fashioned standards,
> > anyway) that would be quite unlikely to evict a guest OS's pages.  Plus
> > I tend to run on shared systems, and deliberately running them out of
> > memory would not be particularly friendly to others using those systems.
> > 
> > I -do- run background scripts that are intended to force the host OS to
> > preempt the guest OSes frequently, but I don't believe that this would
> > cause that bug.
> > 
> > But it seems like it would make more sense to add this sort of thing to
> > whatever KVM tests there are for host-side eviction of guest pages.
>