netdev - Re: [PATCH net v5 1/2] vhost_net: fix ubuf refcount incorrectly when sendmsg fails

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Message-ID: <CA+FuTScnt=jVt2+sagtYUXxTrc7RieKc=YyCdp+0zuS9jCiNuA@mail.gmail.com>
Date:   Sun, 27 Dec 2020 11:14:35 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     wangyunjian <wangyunjian@...wei.com>,
        Network Development <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        virtualization@...ts.linux-foundation.org,
        "Lilijun (Jerry)" <jerry.lilijun@...wei.com>,
        chenchanghu <chenchanghu@...wei.com>,
        xudingke <xudingke@...wei.com>,
        "huangbin (J)" <brian.huangbin@...wei.com>
Subject: Re: [PATCH net v5 1/2] vhost_net: fix ubuf refcount incorrectly when
 sendmsg fails

On Sun, Dec 27, 2020 at 6:26 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Dec 25, 2020 at 03:24:25PM +0800, wangyunjian wrote:
> > From: Yunjian Wang <wangyunjian@...wei.com>
> >
> > Currently the vhost_zerocopy_callback() maybe be called to decrease
> > the refcount when sendmsg fails in tun. The error handling in vhost
> > handle_tx_zerocopy() will try to decrease the same refcount again.
> > This is wrong. To fix this issue, we only call vhost_net_ubuf_put()
> > when vq->heads[nvq->desc].len == VHOST_DMA_IN_PROGRESS.
> >
> > Fixes: 0690899b4d45 ("tun: experimental zero copy tx support")
>
> Are you sure about this tag? the patch in question only affects
> tun, while the fix only affects vhost.

That was my suggestion. But you're right. Perhaps better:

Fixes: bab632d69ee4 ("vhost: vhost TX zero-copy support")

That introduces the actual block that releases the buffer on error:

"
                err = sock->ops->sendmsg(NULL, sock, &msg, len);
                if (unlikely(err < 0)) {
+                       if (zcopy) {
+                               if (ubufs)
+                                       vhost_ubuf_put(ubufs);
+                               vq->upend_idx = ((unsigned)vq->upend_idx - 1) %
+                                       UIO_MAXIOV;
+                       }
"