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

[Stefano Garzarella <sgarzare@redhat.com>]

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@redhat.com> 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@sberdevices.ru>
>>>>> ---
>>>>> 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.

Thanks,
Stefano