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    SubjectRe: WARNING in __mmdrop
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    On 2019/7/29 下午10:44, Michael S. Tsirkin wrote:
    > On Mon, Jul 29, 2019 at 10:24:43PM +0800, Jason Wang wrote:
    >> On 2019/7/29 下午4:59, Michael S. Tsirkin wrote:
    >>> On Mon, Jul 29, 2019 at 01:54:49PM +0800, Jason Wang wrote:
    >>>> On 2019/7/26 下午9:49, Michael S. Tsirkin wrote:
    >>>>>>> Ok, let me retry if necessary (but I do remember I end up with deadlocks
    >>>>>>> last try).
    >>>>>> Ok, I play a little with this. And it works so far. Will do more testing
    >>>>>> tomorrow.
    >>>>>>
    >>>>>> One reason could be I switch to use get_user_pages_fast() to
    >>>>>> __get_user_pages_fast() which doesn't need mmap_sem.
    >>>>>>
    >>>>>> Thanks
    >>>>> OK that sounds good. If we also set a flag to make
    >>>>> vhost_exceeds_weight exit, then I think it will be all good.
    >>>> After some experiments, I came up two methods:
    >>>>
    >>>> 1) switch to use vq->mutex, then we must take the vq lock during range
    >>>> checking (but I don't see obvious slowdown for 16vcpus + 16queues). Setting
    >>>> flags during weight check should work but it still can't address the worst
    >>>> case: wait for the page to be swapped in. Is this acceptable?
    >>>>
    >>>> 2) using current RCU but replace synchronize_rcu() with vhost_work_flush().
    >>>> The worst case is the same as 1) but we can check range without holding any
    >>>> locks.
    >>>>
    >>>> Which one did you prefer?
    >>>>
    >>>> Thanks
    >>> I would rather we start with 1 and switch to 2 after we
    >>> can show some gain.
    >>>
    >>> But the worst case needs to be addressed.
    >>
    >> Yes.
    >>
    >>
    >>> How about sending a signal to
    >>> the vhost thread? We will need to fix up error handling (I think that
    >>> at the moment it will error out in that case, handling this as EFAULT -
    >>> and we don't want to drop packets if we can help it, and surely not
    >>> enter any error states. In particular it might be especially tricky if
    >>> we wrote into userspace memory and are now trying to log the write.
    >>> I guess we can disable the optimization if log is enabled?).
    >>
    >> This may work but requires a lot of changes.
    > I agree.
    >
    >> And actually it's the price of
    >> using vq mutex.
    > Not sure what's meant here.


    I mean if we use vq mutex, it means the critical section was increased
    and we need to deal with swapping then.


    >
    >> Actually, the critical section should be rather small, e.g
    >> just inside memory accessors.
    > Also true.
    >
    >> I wonder whether or not just do synchronize our self like:
    >>
    >> static void inline vhost_inc_vq_ref(struct vhost_virtqueue *vq)
    >> {
    >>         int ref = READ_ONCE(vq->ref);
    >>
    >>         WRITE_ONCE(vq->ref, ref + 1);
    >> smp_rmb();
    >> }
    >>
    >> static void inline vhost_dec_vq_ref(struct vhost_virtqueue *vq)
    >> {
    >>         int ref = READ_ONCE(vq->ref);
    >>
    >> smp_wmb();
    >>         WRITE_ONCE(vq->ref, ref - 1);
    >> }
    >>
    >> static void inline vhost_wait_for_ref(struct vhost_virtqueue *vq)
    >> {
    >>         while (READ_ONCE(vq->ref));
    >> mb();
    >> }
    > Looks good but I'd like to think of a strategy/existing lock that let us
    > block properly as opposed to spinning, that would be more friendly to
    > e.g. the realtime patch.


    Does it make sense to disable preemption in the critical section? Then
    we don't need to block and we have a deterministic time spent on memory
    accssors?


    >
    >> Or using smp_load_acquire()/smp_store_release() instead?
    >>
    >> Thanks
    > These are cheaper on x86, yes.


    Will use this.

    Thanks


    >

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