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

[Stefano Garzarella <sgarzare@redhat.com>]

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@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.
>
>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.

Thanks,
Stefano