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Date:   Fri, 4 Jun 2021 17:03:24 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Arseny Krasnov <arseny.krasnov@kaspersky.com>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jorgen Hansen <jhansen@vmware.com>,
        Norbert Slusarek <nslusarek@gmx.net>,
        Colin Ian King <colin.king@canonical.com>,
        Andra Paraschiv <andraprs@amazon.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "oxffffaa@gmail.com" <oxffffaa@gmail.com>
Subject: Re: [PATCH v10 11/18] virtio/vsock: dequeue callback for
 SOCK_SEQPACKET
Message-ID: <20210604150324.winiikx5h3p6gsyy@steredhat>
References: <20210520191357.1270473-1-arseny.krasnov@kaspersky.com>
 <20210520191801.1272027-1-arseny.krasnov@kaspersky.com>
 <20210603144513.ryjzauq7abnjogu3@steredhat>
 <6b833ccf-ea93-db6a-4743-463ac1cfe817@kaspersky.com>
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In-Reply-To: <6b833ccf-ea93-db6a-4743-463ac1cfe817@kaspersky.com>
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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.

> 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;
>
>>
>>
>