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Date: Fri, 4 Jun 2021 21:03:26 +0300
From: Arseny Krasnov <arseny.krasnov@...persky.com>
To: Stefano Garzarella <sgarzare@...hat.com>
CC: Stefan Hajnoczi <stefanha@...hat.com>,
"Michael S. Tsirkin" <mst@...hat.com>,
Jason Wang <jasowang@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Jorgen Hansen <jhansen@...are.com>,
Norbert Slusarek <nslusarek@....net>,
Colin Ian King <colin.king@...onical.com>,
Andra Paraschiv <andraprs@...zon.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"virtualization@...ts.linux-foundation.org"
<virtualization@...ts.linux-foundation.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"oxffffaa@...il.com" <oxffffaa@...il.com>
Subject: Re: [PATCH v10 11/18] virtio/vsock: dequeue callback for
SOCK_SEQPACKET
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@...persky.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?
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;
>>
>>>
>
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