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

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Message-ID: <wnonuiluxgy6ixoioi57lwlixfgcu27kcewv4ajb3k3hihi773@nv3om2t3tsgo>
Date: Thu, 24 Apr 2025 09:28:50 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Michal Luczaj <mhal@...x.co>
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 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?

>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?

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

Stefano