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. >> 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. 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.
