Re: [RFC PATCH v3 1/9] vsock: SO_RCVLOWAT transport set callback

[Arseniy Krasnov]

On 09.08.2022 13:03, Stefano Garzarella wrote:
> On Tue, Aug 09, 2022 at 09:45:47AM +0000, Arseniy Krasnov wrote:
>> On 09.08.2022 12:37, Arseniy Krasnov wrote:
>>> On 08.08.2022 13:30, Stefano Garzarella wrote:
>>>> On Mon, Aug 8, 2022 at 12:23 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
>>>>>
>>>>> On Wed, Aug 03, 2022 at 01:51:05PM +0000, Arseniy Krasnov wrote:
>>>>>> This adds transport specific callback for SO_RCVLOWAT, because in some
>>>>>> transports it may be difficult to know current available number of bytes
>>>>>> ready to read. Thus, when SO_RCVLOWAT is set, transport may reject it.
>>>>>>
>>>>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
>>>>>> ---
>>>>>> include/net/af_vsock.h   |  1 +
>>>>>> net/vmw_vsock/af_vsock.c | 25 +++++++++++++++++++++++++
>>>>>> 2 files changed, 26 insertions(+)
>>>>>>
>>>>>> diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
>>>>>> index f742e50207fb..eae5874bae35 100644
>>>>>> --- a/include/net/af_vsock.h
>>>>>> +++ b/include/net/af_vsock.h
>>>>>> @@ -134,6 +134,7 @@ struct vsock_transport {
>>>>>>       u64 (*stream_rcvhiwat)(struct vsock_sock *);
>>>>>>       bool (*stream_is_active)(struct vsock_sock *);
>>>>>>       bool (*stream_allow)(u32 cid, u32 port);
>>>>>> +      int (*set_rcvlowat)(struct vsock_sock *, int);
>>>>>
>>>>> checkpatch suggests to add identifier names. For some we put them in,
>>>>> for others we didn't, but I suggest putting them in for the new ones
>>>>> because I think it's clearer too.
>>>>>
>>>>> WARNING: function definition argument 'struct vsock_sock *' should also
>>>>> have an identifier name
>>>>> #25: FILE: include/net/af_vsock.h:137:
>>>>> +       int (*set_rcvlowat)(struct vsock_sock *, int);
>>>>>
>>>>> WARNING: function definition argument 'int' should also have an identifier name
>>>>> #25: FILE: include/net/af_vsock.h:137:
>>>>> +       int (*set_rcvlowat)(struct vsock_sock *, int);
>>>>>
>>>>> total: 0 errors, 2 warnings, 0 checks, 44 lines checked
>>>>>
>>>>>>
>>>>>>       /* SEQ_PACKET. */
>>>>>>       ssize_t (*seqpacket_dequeue)(struct vsock_sock *vsk, struct msghdr *msg,
>>>>>> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>>>>>> index f04abf662ec6..016ad5ff78b7 100644
>>>>>> --- a/net/vmw_vsock/af_vsock.c
>>>>>> +++ b/net/vmw_vsock/af_vsock.c
>>>>>> @@ -2129,6 +2129,30 @@ vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>>>>>>       return err;
>>>>>> }
>>>>>>
>>>>>> +static int vsock_set_rcvlowat(struct sock *sk, int val)
>>>>>> +{
>>>>>> +      const struct vsock_transport *transport;
>>>>>> +      struct vsock_sock *vsk;
>>>>>> +      int err = 0;
>>>>>> +
>>>>>> +      vsk = vsock_sk(sk);
>>>>>> +
>>>>>> +      if (val > vsk->buffer_size)
>>>>>> +              return -EINVAL;
>>>>>> +
>>>>>> +      transport = vsk->transport;
>>>>>> +
>>>>>> +      if (!transport)
>>>>>> +              return -EOPNOTSUPP;
>>>>>
>>>>> I don't know whether it is better in this case to write it in
>>>>> sk->sk_rcvlowat, maybe we can return EOPNOTSUPP only when the trasport
>>>>> is assigned and set_rcvlowat is not defined. This is because usually the
>>>>> options are set just after creation, when the transport is practically
>>>>> unassigned.
>>>>>
>>>>> I mean something like this:
>>>>>
>>>>>          if (transport) {
>>>>>                  if (transport->set_rcvlowat)
>>>>>                          return transport->set_rcvlowat(vsk, val);
>>>>>                  else
>>>>>                          return -EOPNOTSUPP;
>>>>>          }
>>>>>
>>>>>          WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
>>>>>
>>>>>          return 0;
>>>>
>>>> Since hv_sock implements `set_rcvlowat` to return EOPNOTSUPP. maybe we
>>>> can just do the following:
>>>>
>>>>         if (transport && transport->set_rcvlowat)
>>>>                 return transport->set_rcvlowat(vsk, val);
>>>>
>>>>         WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
>>>>         return 0;
>>>>
>>>> That is, the default behavior is to set sk->sk_rcvlowat, but for
>>>> transports that want a different behavior, they need to define
>>>> set_rcvlowat() (like hv_sock).
>>> Hm ok, i see. I've implemented logic when non-empty transport is required, because hyperv transport
>>> forbids to set SO_RCVLOWAT, so user needs to call this setsockopt AFTER transport is assigned(to check
>>> that transport allows it. Not after socket creation as You mentioned above). Otherwise there is no sense
>>> in such callback - it will be never used. Also in code above - for hyperv we will have different behavior
>>> depends on when set_rcvlowat is called: before or after transport assignment. Is it ok?
>> sorry, i mean: for hyperv, if user sets sk_rcvlowat before transport is assigned, it sees 0 - success, but in fact
>> hyperv transport forbids this option.
> 
> I see, but I think it's better to set it and not respect in hyperv (as we've practically done until now with all transports) than to prevent the setting until we assign a transport.
> 
> At most when we use hyperv anyway we get notified per byte, so we should just get more notifications than we expect.
see it, ok, thanks
> 
> Thanks,
> Stefano
>