Subject: Re: [PATCH] vhost: clean up outstanding buffers before setting vring

From: Shirley Ma <[email-masked]>
Date: Tue, 19 Jul 2011 13:50:53 -0700

On Tue, 2011-07-19 at 22:49 +0300, Michael S. Tsirkin wrote:
> On Tue, Jul 19, 2011 at 11:02:26AM -0700, Shirley Ma wrote:
> > The outstanding DMA buffers need to be clean up before setting vring
> in
> > vhost. Otherwise the vring would be out of sync.
> > 
> > Signed-off-by: Shirley Ma<[email-masked]>
> 
> I suspect what is missing is calling
> vhost_zerocopy_signal_used then?
> 
> If yes we probably should do it after
> changing the backend, not on vring set.

I think vhost_zerocopy_signal_used might not be sufficient. But we can
test it out by remove/reloading the guest virtio_net module.

> > ---
> > 
> >  drivers/vhost/vhost.c |   11 +++++++++--
> >  1 files changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index c14c42b..d6315b4 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -445,8 +445,10 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >                       vhost_poll_flush(&dev->vqs[i].poll);
> >               }
> >               /* Wait for all lower device DMAs done. */
> > -             if (dev->vqs[i].ubufs)
> > +             if (dev->vqs[i].ubufs) {
> >                       vhost_ubuf_put_and_wait(dev->vqs[i].ubufs);
> > +                     kfree(dev->vqs[i].ubufs);
> > +             }
> >  
> >               /* Signal guest as appropriate. */
> >               vhost_zerocopy_signal_used(&dev->vqs[i]);
> > @@ -651,6 +653,12 @@ static long vhost_set_vring(struct vhost_dev
> *d, int ioctl, void __user *argp)
> >       vq = d->vqs + idx;
> >  
> >       mutex_lock(&vq->mutex);
> > +     /* Wait for all lower device DMAs done. */
> > +     if (vq->ubufs)
> > +             vhost_ubuf_put_and_wait(vq->ubufs);
> 
> Could you elaborate on the problem you observe please?
> At least in theory, existing code flushes outstanding
> requests when backend is changed.
> And since vring set verifies no backend is active,
> we should be fine?

The problem encounters when guest rmmod virtio_net module, then reload
the module, and configure the interface, it complains about some ring id
is not a head. With this patch, the problem is solved. 

> 
> > +
> > +     /* Signal guest as appropriate. */
> > +     vhost_zerocopy_signal_used(vq);
> >  
> >       switch (ioctl) {
> >       case VHOST_SET_VRING_NUM:
> > @@ -1592,7 +1600,6 @@ void vhost_ubuf_put_and_wait(struct
> vhost_ubuf_ref *ubufs)
> >  {
> >       kref_put(&ubufs->kref, vhost_zerocopy_done_signal);
> >       wait_event(ubufs->wait, !atomic_read(&ubufs->kref.refcount));
> > -     kfree(ubufs);
> 
> Won't this leak memory when ubufs are switched in
> vhost_net_set_backend? 

Right, I forgot to check net.c, whenever it calls
vhot_ubuf_put_and_wait, it should call kfree(ubufs).