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

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Jun 07, 2021 at 04:18:38PM +0300, Arseny Krasnov wrote:
>
>On 07.06.2021 14:04, Stefano Garzarella wrote:
>> On Fri, Jun 04, 2021 at 09:03:26PM +0300, Arseny Krasnov wrote:
>>> 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@kaspersky.com>
>>>>>>> ---
>>>>>>> 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?
>>>
>> I think VIRTIO_VSOCK_SEQ_BEGIN is redundant, using only EOR should be
>> fine.
>>
>> When we receive EOR we know that this is the last packet on this message
>> and the next packet will be the first of a new message.
>>
>> What we should do is check that we have all the fragments of a packet
>> and return them all together, otherwise we have to say we have nothing.
>>
>> For example as we process packets from the vitqueue and queue them in
>> the rx_queue we could use a counter of how many EORs are in the
>> rx_queue, which we decrease in virtio_transport_seqpacket_do_dequeue()
>> when we copied all the fragments.
>>
>> If the counter is 0, we don't remove anything from the queue and
>> virtio_transport_seqpacket_do_dequeue() returns 0.
>>
>> So .seqpacket_dequeue should return 0 if there is not at least one
>> complete message, or return the entire message. A partial message should
>> never return.
>>
>> What do you think?
>
>I like it, i've implemented this approach in some early pre v1 versions.
>
>But in this case, credit update logic will be changed - in current implementation
>
>(both seqpacket and stream) credit update reply is sent when data is copied
>
>to user's buffer(e.g. we copy data somewhere, free packet and ready to process
>
>new packet). But if we don't touch user's buffer and keeping incoming packet in rx queue
>
>until whole record is ready, when to send credit update?

I think the best approach could be to send credit updates when we remove 
them from the rx_queue.

Stefano