Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

[Koichiro Den <den@klaipeden.com>]

On Tue, 2017-08-22 at 13:19 -0400, Willem de Bruijn wrote:
> > > > >         /* Don't wait up for transmitted skbs to be freed. */
> > > > >         if (!use_napi) {
> > > > > +               if (skb_shinfo(skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
> > > > > +                       struct ubuf_info *uarg;
> > > > > +                       uarg = skb_shinfo(skb)->destructor_arg;
> > > > > +                       if (uarg->callback)
> > > > > +                           uarg->callback(uarg, true);
> > > > > +                       skb_shinfo(skb)->destructor_arg = NULL;
> > > > > +                       skb_shinfo(skb)->tx_flags &=
> > > > > ~SKBTX_DEV_ZEROCOPY;
> > > > > +               }
> > > > 
> > > > Instead of open coding, this can use skb_zcopy_clear.
> > > 
> > > It is not correct to only send the zerocopy completion here, as
> > > the skb will still be shared with the nic. The only safe approach
> > > to notifying early is to create a copy with skb_orphan_frags_rx.
> > > That will call skb_zcopy_clear(skb, false) internally. But the
> > > copy will be very expensive.
> > 
> > I noticed this email after my last post. I cannot not imagine how it could
> > be
> > unsafe in virtio case. Sorry could you explain a bit more?
> 
> A guest process sends a packet with MSG_ZEROCOPY to the
> virtio-net device. As soon as the process receives the completion
> notification, it is allowed to reuse the memory backing the packet.
> 
> A call to skb_zcopy_clear in virtio-net start_xmit will notify the
> process that it is allowed to reuse the memory. But the user pages
> are still linked into the skb frags and are about to be shared with
> the host os.
> 
> > > Is the deadlock you refer to the case where tx interrupts are
> > > disabled, so transmit completions are only handled in start_xmit
> > > and somehow the socket(s) are unable to send new data? The
> > > question is what is blocking them. If it is zerocopy, it may be a
> > > too low optmem_max or hitting the per-user locked pages limit.
> > > We may have to keep interrupts enabled when zerocopy skbs
> > > are in flight.
> > 
> > Even if we keep interrupts enabled, We miss the completion without
> > start_xmit.
> > And it's also likely that the next start_xmit depends on the completion
> > itself
> > as I described in my last post.
> > 
Thanks for the explanation, I misinterpreted the "nic" part, now it's clear.
Sorry about bothering.