Re: [PATCH v10 11/18] virtio/vsock: dequeue callback for SOCK_SEQPACKET

[Arseny Krasnov]

On 04.06.2021 18:03, Stefano Garzarella wrote:
> On Fri, Jun 04, 2021 at 04:12:23PM +0300, Arseny Krasnov wrote:
>> On 03.06.2021 17:45, Stefano Garzarella wrote:
>>> On Thu, May 20, 2021 at 10:17:58PM +0300, Arseny Krasnov wrote:
>>>> Callback fetches RW packets from rx queue of socket until whole record
>>>> is copied(if user's buffer is full, user is not woken up). This is done
>>>> to not stall sender, because if we wake up user and it leaves syscall,
>>>> nobody will send credit update for rest of record, and sender will wait
>>>> for next enter of read syscall at receiver's side. So if user buffer is
>>>> full, we just send credit update and drop data.
>>>>
>>>> Signed-off-by: Arseny Krasnov <arseny.krasnov@xxxxxxxxxxxxx>
>>>> ---
>>>> v9 -> v10:
>>>> 1) Number of dequeued bytes incremented even in case when
>>>>    user's buffer is full.
>>>> 2) Use 'msg_data_left()' instead of direct access to 'msg_hdr'.
>>>> 3) Rename variable 'err' to 'dequeued_len', in case of error
>>>>    it has negative value.
>>>>
>>>> include/linux/virtio_vsock.h            |  5 ++
>>>> net/vmw_vsock/virtio_transport_common.c | 65 +++++++++++++++++++++++++
>>>> 2 files changed, 70 insertions(+)
>>>>
>>>> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>>>> index dc636b727179..02acf6e9ae04 100644
>>>> --- a/include/linux/virtio_vsock.h
>>>> +++ b/include/linux/virtio_vsock.h
>>>> @@ -80,6 +80,11 @@ virtio_transport_dgram_dequeue(struct vsock_sock *vsk,
>>>> 			       struct msghdr *msg,
>>>> 			       size_t len, int flags);
>>>>
>>>> +ssize_t
>>>> +virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
>>>> +				   struct msghdr *msg,
>>>> +				   int flags,
>>>> +				   bool *msg_ready);
>>>> s64 virtio_transport_stream_has_data(struct vsock_sock *vsk);
>>>> s64 virtio_transport_stream_has_space(struct vsock_sock *vsk);
>>>>
>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>> index ad0d34d41444..61349b2ea7fe 100644
>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>> @@ -393,6 +393,59 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>> 	return err;
>>>> }
>>>>
>>>> +static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
>>>> +						 struct msghdr *msg,
>>>> +						 int flags,
>>>> +						 bool *msg_ready)
>>>> +{
>>>> +	struct virtio_vsock_sock *vvs = vsk->trans;
>>>> +	struct virtio_vsock_pkt *pkt;
>>>> +	int dequeued_len = 0;
>>>> +	size_t user_buf_len = msg_data_left(msg);
>>>> +
>>>> +	*msg_ready = false;
>>>> +	spin_lock_bh(&vvs->rx_lock);
>>>> +
>>>> +	while (!*msg_ready && !list_empty(&vvs->rx_queue) && dequeued_len >= 0) {
>>> I'
>>>
>>>> +		size_t bytes_to_copy;
>>>> +		size_t pkt_len;
>>>> +
>>>> +		pkt = list_first_entry(&vvs->rx_queue, struct virtio_vsock_pkt, list);
>>>> +		pkt_len = (size_t)le32_to_cpu(pkt->hdr.len);
>>>> +		bytes_to_copy = min(user_buf_len, pkt_len);
>>>> +
>>>> +		if (bytes_to_copy) {
>>>> +			/* sk_lock is held by caller so no one else can dequeue.
>>>> +			 * Unlock rx_lock since memcpy_to_msg() may sleep.
>>>> +			 */
>>>> +			spin_unlock_bh(&vvs->rx_lock);
>>>> +
>>>> +			if (memcpy_to_msg(msg, pkt->buf, bytes_to_copy))
>>>> +				dequeued_len = -EINVAL;
>>> I think here is better to return the error returned by memcpy_to_msg(),
>>> as we do in the other place where we use memcpy_to_msg().
>>>
>>> I mean something like this:
>>> 			err = memcpy_to_msgmsg, pkt->buf, bytes_to_copy);
>>> 			if (err)
>>> 				dequeued_len = err;
>> Ack
>>>> +			else
>>>> +				user_buf_len -= bytes_to_copy;
>>>> +
>>>> +			spin_lock_bh(&vvs->rx_lock);
>>>> +		}
>>>> +
>>> Maybe here we can simply break the cycle if we have an error:
>>> 		if (dequeued_len < 0)
>>> 			break;
>>>
>>> Or we can refactor a bit, simplifying the while() condition and also the
>>> code in this way (not tested):
>>>
>>> 	while (!*msg_ready && !list_empty(&vvs->rx_queue)) {
>>> 		...
>>>
>>> 		if (bytes_to_copy) {
>>> 			int err;
>>>
>>> 			/* ...
>>> 			*/
>>> 			spin_unlock_bh(&vvs->rx_lock);
>>> 			err = memcpy_to_msgmsg, pkt->buf, bytes_to_copy);
>>> 			if (err) {
>>> 				dequeued_len = err;
>>> 				goto out;
>>> 			}
>>> 			spin_lock_bh(&vvs->rx_lock);
>>>
>>> 			user_buf_len -= bytes_to_copy;
>>> 		}
>>>
>>> 		dequeued_len += pkt_len;
>>>
>>> 		if (le32_to_cpu(pkt->hdr.flags) & VIRTIO_VSOCK_SEQ_EOR)
>>> 			*msg_ready = true;
>>>
>>> 		virtio_transport_dec_rx_pkt(vvs, pkt);
>>> 		list_del(&pkt->list);
>>> 		virtio_transport_free_pkt(pkt);
>>> 	}
>>>
>>> out:
>>> 	spin_unlock_bh(&vvs->rx_lock);
>>>
>>> 	virtio_transport_send_credit_update(vsk);
>>>
>>> 	return dequeued_len;
>>> }
>> I think we can't do 'goto out' or break, because in case of error, we still need
>> to free packet.
> Didn't we have code that remove packets from a previous message?
> I don't see it anymore.
>
> For example if we have 10 packets queued for a message (the 10th packet 
> has the EOR flag) and the memcpy_to_msg() fails on the 2nd packet, with 
> you proposal we are freeing only the first 2 packets, the rest is there 
> and should be freed when reading the next message, but I don't see that 
> code.
>
> The same can happen if the recvmsg syscall is interrupted. In that case 
> we report that nothing was copied, but we freed the first N packets, so 
> they are lost but the other packets are still in the queue.
>
> Please check also the patch where we implemented 
> __vsock_seqpacket_recvmsg().
>
> I thinks we should free packets only when we are sure we copied them to 
> the user space.

Hm, yes, this is problem. To solve it i can restore previous approach

with seqbegin/seqend. In that case i can detect unfinished record and

drop it's packets. Seems seqbegin will be a bit like VIRTIO_VSOCK_SEQ_EOR in flags

field of header(e.g. VIRTIO_VSOCK_SEQ_BEGIN). Message id and length are unneeded,

as channel considedered lossless. What do You think?


Thank You

>
>> It is possible to do something like this:
>>
>> 		virtio_transport_dec_rx_pkt(vvs, pkt);
>> 		list_del(&pkt->list);
>> 		virtio_transport_free_pkt(pkt);
>>
>> 		if (dequeued_len < 0)
>> 			break;
>>
>>>
>