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. >
