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

[Arseny Krasnov]

On 08.06.2021 13:19, Stefano Garzarella wrote:
> On Tue, Jun 08, 2021 at 12:40:39PM +0300, Arseny Krasnov wrote:
>> On 08.06.2021 11:23, Stefano Garzarella wrote:
>>> 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@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?
>>>>>>
>>>>> 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.
>> In that case, it will be impossible to send message bigger than size of rx buffer
>>
>> (e.g. credit allowed size), because packet will be queued without credit update
>>
>> reply until credit allowed reach 0.
>>
> Yep, but I think it is a reasonable limit for a datagram socket.
>
> Maybe we can add a check on the TX side, since we know this value and 
> return an error to the user.

E.g., to before sending message  using SEQPACKET socket,

i need to call setsockopt with SO_VM_SOCKETS_BUFFER_MAX_SIZE/

SO_VM_SOCKETS_BUFFER_SIZE params to setup maximum message size,

if user tries to send message bigger than it, return -EMSGSIZE ?


Thank You

>
> Thanks,
> Stefano
>
>