linux-kernel - Re: [PATCH net-next v2 1/3] vsock: Linger on unsent data

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Message-ID: <81940d67-1a9b-42e1-8594-33af86397df6@rbox.co>
Date: Thu, 24 Apr 2025 13:24:59 +0200
From: Michal Luczaj <mhal@...x.co>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Luigi Leonardi <leonardi@...hat.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, "Michael S. Tsirkin" <mst@...hat.com>,
 Jason Wang <jasowang@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 Stefan Hajnoczi <stefanha@...hat.com>, virtualization@...ts.linux.dev,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/3] vsock: Linger on unsent data

On 4/24/25 10:36, Stefano Garzarella wrote:
> On Thu, 24 Apr 2025 at 09:53, Michal Luczaj <mhal@...x.co> wrote:
>>
>> On 4/24/25 09:28, Stefano Garzarella wrote:
>>> On Wed, Apr 23, 2025 at 11:06:33PM +0200, Michal Luczaj wrote:
>>>> On 4/23/25 18:34, Stefano Garzarella wrote:
>>>>> On Wed, Apr 23, 2025 at 05:53:12PM +0200, Luigi Leonardi wrote:
>>>>>> Hi Michal,
>>>>>>
>>>>>> On Mon, Apr 21, 2025 at 11:50:41PM +0200, Michal Luczaj wrote:
>>>>>>> Currently vsock's lingering effectively boils down to waiting (or timing
>>>>>>> out) until packets are consumed or dropped by the peer; be it by receiving
>>>>>>> the data, closing or shutting down the connection.
>>>>>>>
>>>>>>> To align with the semantics described in the SO_LINGER section of man
>>>>>>> socket(7) and to mimic AF_INET's behaviour more closely, change the logic
>>>>>>> of a lingering close(): instead of waiting for all data to be handled,
>>>>>>> block until data is considered sent from the vsock's transport point of
>>>>>>> view. That is until worker picks the packets for processing and decrements
>>>>>>> virtio_vsock_sock::bytes_unsent down to 0.
>>>>>>>
>>>>>>> Note that such lingering is limited to transports that actually implement
>>>>>>> vsock_transport::unsent_bytes() callback. This excludes Hyper-V and VMCI,
>>>>>>> under which no lingering would be observed.
>>>>>>>
>>>>>>> The implementation does not adhere strictly to man page's interpretation of
>>>>>>> SO_LINGER: shutdown() will not trigger the lingering. This follows AF_INET.
>>>>>>>
>>>>>>> Signed-off-by: Michal Luczaj <mhal@...x.co>
>>>>>>> ---
>>>>>>> net/vmw_vsock/virtio_transport_common.c | 13 +++++++++++--
>>>>>>> 1 file changed, 11 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>>>>> index 7f7de6d8809655fe522749fbbc9025df71f071bd..aeb7f3794f7cfc251dde878cb44fdcc54814c89c 100644
>>>>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>>>>> @@ -1196,12 +1196,21 @@ static void virtio_transport_wait_close(struct sock *sk, long timeout)
>>>>>>> {
>>>>>>>   if (timeout) {
>>>>>>>           DEFINE_WAIT_FUNC(wait, woken_wake_function);
>>>>>>> +         ssize_t (*unsent)(struct vsock_sock *vsk);
>>>>>>> +         struct vsock_sock *vsk = vsock_sk(sk);
>>>>>>> +
>>>>>>> +         /* Some transports (Hyper-V, VMCI) do not implement
>>>>>>> +          * unsent_bytes. For those, no lingering on close().
>>>>>>> +          */
>>>>>>> +         unsent = vsk->transport->unsent_bytes;
>>>>>>> +         if (!unsent)
>>>>>>> +                 return;
>>>>>>
>>>>>> IIUC if `unsent_bytes` is not implemented, virtio_transport_wait_close
>>>>>> basically does nothing. My concern is that we are breaking the
>>>>>> userspace due to a change in the behavior: Before this patch, with a
>>>>>> vmci/hyper-v transport, this function would wait for SOCK_DONE to be
>>>>>> set, but not anymore.
>>>>>
>>>>> Wait, we are in virtio_transport_common.c, why we are talking about
>>>>> Hyper-V and VMCI?
>>>>>
>>>>> I asked to check `vsk->transport->unsent_bytes` in the v1, because this
>>>>> code was part of af_vsock.c, but now we are back to virtio code, so I'm
>>>>> confused...
>>>>
>>>> Might your confusion be because of similar names?
>>>
>>> In v1 this code IIRC was in af_vsock.c, now you pushed back on virtio
>>> common code, so I still don't understand how
>>> virtio_transport_wait_close() can be called with vmci or hyper-v
>>> transports.
>>>
>>> Can you provide an example?
>>
>> You're right, it was me who was confused. VMCI and Hyper-V have their own
>> vsock_transport::release callbacks that do not call
>> virtio_transport_wait_close().
>>
>> So VMCI and Hyper-V never lingered anyway?
> 
> I think so.
> 
> Indeed I was happy with v1, since I think this should be supported by
> the vsock core and should not depend on the transport.
> But we can do also later.

OK, for now let me fix this nonsense in comment and commit message.

But I'll wait for your opinion on [1] (drop, squash, change order of
patches?) before posting v3.

[1]:
https://lore.kernel.org/netdev/20250421-vsock-linger-v2-2-fe9febd64668@rbox.co/

Thanks,
Michal