Re: [PATCH v2 5/5] vhost: add device started check in migration set log

[Li Feng]

Hi, Dima.

If vhost_migration_log return < 0, then vhost_log_global_start will
trigger a crash.
Does your patch have process this abort?
If a disconnect happens in the migration stage, the correct operation
is to stop the migration, right?

 841 static void vhost_log_global_start(MemoryListener *listener)
 842 {
 843     int r;
 844
 845     r = vhost_migration_log(listener, true);
 846     if (r < 0) {
 847         abort();
 848     }
 849 }

Thanks,

Feng Li

Jason Wang <address@hidden> 于2020年5月12日周二 上午11:33写道：
>
>
> On 2020/5/11 下午5:25, Dima Stepanov wrote:
> > On Mon, May 11, 2020 at 11:15:53AM +0800, Jason Wang wrote:
> >> On 2020/4/30 下午9:36, Dima Stepanov wrote:
> >>> If vhost-user daemon is used as a backend for the vhost device, then we
> >>> should consider a possibility of disconnect at any moment. If such
> >>> disconnect happened in the vhost_migration_log() routine the vhost
> >>> device structure will be clean up.
> >>> At the start of the vhost_migration_log() function there is a check:
> >>>    if (!dev->started) {
> >>>        dev->log_enabled = enable;
> >>>        return 0;
> >>>    }
> >>> To be consistent with this check add the same check after calling the
> >>> vhost_dev_set_log() routine. This in general help not to break a
> >>> migration due the assert() message. But it looks like that this code
> >>> should be revised to handle these errors more carefully.
> >>>
> >>> In case of vhost-user device backend the fail paths should consider the
> >>> state of the device. In this case we should skip some function calls
> >>> during rollback on the error paths, so not to get the NULL dereference
> >>> errors.
> >>>
> >>> Signed-off-by: Dima Stepanov <address@hidden>
> >>> ---
> >>>   hw/virtio/vhost.c | 39 +++++++++++++++++++++++++++++++++++----
> >>>   1 file changed, 35 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>> index 3ee50c4..d5ab96d 100644
> >>> --- a/hw/virtio/vhost.c
> >>> +++ b/hw/virtio/vhost.c
> >>> @@ -787,6 +787,17 @@ static int vhost_dev_set_features(struct vhost_dev 
> >>> *dev,
> >>>   static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
> >>>   {
> >>>       int r, i, idx;
> >>> +
> >>> +    if (!dev->started) {
> >>> +        /*
> >>> +         * If vhost-user daemon is used as a backend for the
> >>> +         * device and the connection is broken, then the vhost_dev
> >>> +         * structure will be reset all its values to 0.
> >>> +         * Add additional check for the device state.
> >>> +         */
> >>> +        return -1;
> >>> +    }
> >>> +
> >>>       r = vhost_dev_set_features(dev, enable_log);
> >>>       if (r < 0) {
> >>>           goto err_features;
> >>> @@ -801,12 +812,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, 
> >>> bool enable_log)
> >>>       }
> >>>       return 0;
> >>>   err_vq:
> >>> -    for (; i >= 0; --i) {
> >>> +    /*
> >>> +     * Disconnect with the vhost-user daemon can lead to the
> >>> +     * vhost_dev_cleanup() call which will clean up vhost_dev
> >>> +     * structure.
> >>> +     */
> >>> +    for (; dev->started && (i >= 0); --i) {
> >>>           idx = dev->vhost_ops->vhost_get_vq_index(
> >>
> >> Why need the check of dev->started here, can started be modified outside
> >> mainloop? If yes, I don't get the check of !dev->started in the beginning 
> >> of
> >> this function.
> >>
> > No dev->started can't change outside the mainloop. The main problem is
> > only for the vhost_user_blk daemon. Consider the case when we
> > successfully pass the dev->started check at the beginning of the
> > function, but after it we hit the disconnect on the next call on the
> > second or third iteration:
> >       r = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx, enable_log);
> > The unix socket backend device will call the disconnect routine for this
> > device and reset the structure. So the structure will be reset (and
> > dev->started set to false) inside this set_addr() call.
>
>
> I still don't get here. I think the disconnect can not happen in the
> middle of vhost_dev_set_log() since both of them were running in
> mainloop. And even if it can, we probably need other synchronization
> mechanism other than simple check here.
>
>
> >   So
> > we shouldn't call the clean up calls because this virtqueues were clean
> > up in the disconnect call. But we should protect these calls somehow, so
> > it will not hit SIGSEGV and we will be able to pass migration.
> >
> > Just to summarize it:
> > For the vhost-user-blk devices we ca hit clean up calls twice in case of
> > vhost disconnect:
> > 1. The first time during the disconnect process. The clean up is called
> > inside it.
> > 2. The second time during roll back clean up.
> > So if it is the case we should skip p2.
> >
> >>> dev, dev->vq_index + i);
> >>>           vhost_virtqueue_set_addr(dev, dev->vqs + i, idx,
> >>>                                    dev->log_enabled);
> >>>       }
> >>> -    vhost_dev_set_features(dev, dev->log_enabled);
> >>> +    if (dev->started) {
> >>> +        vhost_dev_set_features(dev, dev->log_enabled);
> >>> +    }
> >>>   err_features:
> >>>       return r;
> >>>   }
> >>> @@ -832,7 +850,15 @@ static int vhost_migration_log(MemoryListener 
> >>> *listener, int enable)
> >>>       } else {
> >>>           vhost_dev_log_resize(dev, vhost_get_log_size(dev));
> >>>           r = vhost_dev_set_log(dev, true);
> >>> -        if (r < 0) {
> >>> +        /*
> >>> +         * The dev log resize can fail, because of disconnect
> >>> +         * with the vhost-user-blk daemon. Check the device
> >>> +         * state before calling the vhost_dev_set_log()
> >>> +         * function.
> >>> +         * Don't return error if device isn't started to be
> >>> +         * consistent with the check above.
> >>> +         */
> >>> +        if (dev->started && r < 0) {
> >>>               return r;
> >>>           }
> >>>       }
> >>> @@ -1739,7 +1765,12 @@ int vhost_dev_start(struct vhost_dev *hdev, 
> >>> VirtIODevice *vdev)
> >>>   fail_log:
> >>>       vhost_log_put(hdev, false);
> >>>   fail_vq:
> >>> -    while (--i >= 0) {
> >>> +    /*
> >>> +     * Disconnect with the vhost-user daemon can lead to the
> >>> +     * vhost_dev_cleanup() call which will clean up vhost_dev
> >>> +     * structure.
> >>> +     */
> >>> +    while ((--i >= 0) && (hdev->started)) {
> >>>           vhost_virtqueue_stop(hdev,
> >>>                                vdev,
> >>>                                hdev->vqs + i,
> >>
> >> This should be a separate patch.
> > Do you mean i should split this patch to two patches?
>
>
> Yes.
>
> Thanks
>
>
> >
> > Thanks.
> >
> >> Thanks
> >>
>