Re: [PATCH v11 11/18] virtio/vsock: dequeue callback for SOCK_SEQPACKET

Stefano Garzarella
2021-06-18T15:56:06Z

On Fri, Jun 18, 2021 at 06:04:37PM +0300, Arseny Krasnov wrote:
>
>On 18.06.2021 16:44, Stefano Garzarella wrote:
>> Hi Arseny,
>> the series looks great, I have just a question below about
>> seqpacket_dequeue.
>>
>> I also sent a couple a simple fixes, it would be great if you can review
>> them:
>> https://lore.kernel.org/netdev/20210618133526.300347-1-sgarzare@redhat.com/
>>
>>
>> On Fri, Jun 11, 2021 at 02:12:38PM +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>
>>> ---
>>> v10 -> v11:
>>> 1) 'msg_count' field added to count current number of EORs.
>>> 2) 'msg_ready' argument removed from callback.
>>> 3) If 'memcpy_to_msg()' failed during copy loop, there will be
>>> no next attempts to copy data, rest of record will be freed.
>>>
>>> include/linux/virtio_vsock.h | 5 ++
>>> net/vmw_vsock/virtio_transport_common.c | 84 +++++++++++++++++++++++++
>>> 2 files changed, 89 insertions(+)
>>>
>>> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>>> index dc636b727179..1d9a302cb91d 100644
>>> --- a/include/linux/virtio_vsock.h
>>> +++ b/include/linux/virtio_vsock.h
>>> @@ -36,6 +36,7 @@ struct virtio_vsock_sock {
>>> u32 rx_bytes;
>>> u32 buf_alloc;
>>> struct list_head rx_queue;
>>> + u32 msg_count;
>>> };
>>>
>>> struct virtio_vsock_pkt {
>>> @@ -80,6 +81,10 @@ 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);
>>> 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..1e1df19ec164 100644
>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>> @@ -393,6 +393,78 @@ 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)
>>> +{
>>> + 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);
>>> + bool copy_failed = false;
>>> + bool msg_ready = false;
>>> +
>>> + spin_lock_bh(&vvs->rx_lock);
>>> +
>>> + if (vvs->msg_count == 0) {
>>> + spin_unlock_bh(&vvs->rx_lock);
>>> + return 0;
>>> + }
>>> +
>>> + while (!msg_ready) {
>>> + pkt = list_first_entry(&vvs->rx_queue, struct virtio_vsock_pkt, list);
>>> +
>>> + if (!copy_failed) {
>>> + size_t pkt_len;
>>> + size_t bytes_to_copy;
>>> +
>>> + pkt_len = (size_t)le32_to_cpu(pkt->hdr.len);
>>> + bytes_to_copy = min(user_buf_len, pkt_len);
>>> +
>>> + if (bytes_to_copy) {
>>> + int err;
>>> +
>>> + /* 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);
>>> +
>>> + err = memcpy_to_msg(msg, pkt->buf, bytes_to_copy);
>>> + if (err) {
>>> + /* Copy of message failed, set flag to skip
>>> + * copy path for rest of fragments. Rest of
>>> + * fragments will be freed without copy.
>>> + */
>>> + copy_failed = true;
>>> + dequeued_len = err;
>> If we fail to copy the message we will discard the entire packet.
>> Is it acceptable for the user point of view, or we should leave the
>> packet in the queue and the user can retry, maybe with a different
>> buffer?
>>
>> Then we can remove the packets only when we successfully copied all the
>> fragments.
>>
>> I'm not sure make sense, maybe better to check also other
>> implementations :-)
>>
>> Thanks,
>> Stefano
>
>Understand, i'll check it on weekend, anyway I think it is
>not critical for implementation.

Yep, I agree.

>
>
>I have another question: may be it is useful to research for
>approach where packets are not queued until whole message
>is received, but copied to user's buffer thus freeing memory.
>(like previous implementation, of course with solution of problem
>where part of message still in queue, while reader was woken
>by timeout or signal).
>
>I think it is better, because  in current version, sender may set
>'peer_alloc_buf' to  for example 1MB, so at receiver we get
>1MB of 'kmalloc()' memory allocated, while having user's buffer
>to copy data there or drop it(if user's buffer is full). This way
>won't change spec(e.g. no message id or SEQ_BEGIN will be added).
>
>What do You think?

Yep, I see your point and it would be great, but I think the main issues
to fix is how to handle a signal while we are waiting other fragments
since the other peer can take unspecified time to send them.

Note that the 'peer_alloc_buf' in the sender, is the value get from the
receiver, so if the receiver doesn't want to allocate 1MB, can advertise
a small buffer size.

Thanks,
Stefano