On Sun, Dec 13, 2020 at 10:30 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
>
> On 2020/12/14 上午9:32, Willem de Bruijn wrote:
> > On Sat, Dec 12, 2020 at 7:18 PM Willem de Bruijn
> > <willemdebruijn.kernel@xxxxxxxxx> wrote:
> >>>>> afterwards, the error handling in vhost handle_tx() will try to
> >>>>> decrease the same refcount again. This is wrong and fix this by delay
> >>>>> copying ubuf_info until we're sure there's no errors.
> >>>> I think the right approach is to address this in the error paths, rather than
> >>>> complicate the normal datapath.
> >>>>
> >>>> Is it sufficient to suppress the call to vhost_net_ubuf_put in the handle_tx
> >>>> sendmsg error path, given that vhost_zerocopy_callback will be called on
> >>>> kfree_skb?
> >>> We can not call kfree_skb() until the skb was created.
> >>>
> >>>> Or alternatively clear the destructor in drop:
> >>> The uarg->callback() is called immediately after we decide do datacopy
> >>> even if caller want to do zerocopy. If another error occurs later, the vhost
> >>> handle_tx() will try to decrease it again.
> >> Oh right, I missed the else branch in this path:
> >>
> >> /* copy skb_ubuf_info for callback when skb has no error */
> >> if (zerocopy) {
> >> skb_shinfo(skb)->destructor_arg = msg_control;
> >> skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
> >> skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> >> } else if (msg_control) {
> >> struct ubuf_info *uarg = msg_control;
> >> uarg->callback(uarg, false);
> >> }
> >>
> >> So if handle_tx_zerocopy calls tun_sendmsg with ubuf_info (and thus a
> >> reference to release), there are these five options:
> >>
> >> 1. tun_sendmsg succeeds, ubuf_info is associated with skb.
> >> reference released from kfree_skb calling vhost_zerocopy_callback later
> >>
> >> 2. tun_sendmsg succeeds, ubuf_info is released immediately, as skb is
> >> not zerocopy.
> >>
> >> 3. tun_sendmsg fails before creating skb, handle_tx_zerocopy correctly
> >> cleans up on receiving error from tun_sendmsg.
> >>
> >> 4. tun_sendmsg fails after creating skb, but with copying: decremented
> >> at branch shown above + again in handle_tx_zerocopy
> >>
> >> 5. tun_sendmsg fails after creating skb, with zerocopy: decremented at
> >> kfree_skb in drop: + again in handle_tx_zerocopy
> >>
> >> Since handle_tx_zerocopy has no idea whether on error 3, 4 or 5
> >> occurred,
> > Actually, it does. If sendmsg returns an error, it can test whether
> > vq->heads[nvq->upend_idx].len != VHOST_DMA_IN_PROGRESS.
>
>
> Just to make sure I understand this. Any reason for it can't be
> VHOST_DMA_IN_PROGRESS here?
It can be, and it will be if tun_sendmsg returns EINVAL before
assigning the skb destructor.
Only if tun_sendmsg released the zerocopy state through
kfree_skb->vhost_zerocopy_callback will it have been updated to
VHOST_DMA_DONE_LEN. And only then must the caller not try to release
the state again.
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