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

    From Paolo Bonzini@21:1/5 to Paul E. McKenney on Mon Oct 2 14:50:03 2017
    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.

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