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

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Message-ID: <5a4f8925-0e4d-4e4c-9230-6c69af179d3e@rbox.co>
Date: Thu, 24 Apr 2025 09:52:47 +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 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?

>> vsock_transport::unsent_bytes != virtio_vsock_sock::bytes_unsent
>>
>> I agree with Luigi, it is a breaking change for userspace depending on a
>> non-standard behaviour. What's the protocol here; do it anyway, then see if
>> anyone complains?
>>
>> As for Hyper-V and VMCI losing the "lingering", do we care? And if we do,
>> take Hyper-V, is it possible to test any changes without access to
>> proprietary host/hypervisor?
>>
> 
> Again, how this code can be called when using vmci or hyper-v 
> transports?

It cannot, you're right.

> If we go back on v1 implementation, I can understand it, but with this 
> version I really don't understand the scenario.
> 
> Stefano
>