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

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Message-ID: <ee09df9b-9804-49de-b43b-99ccd4cbe742@rbox.co>
Date: Wed, 23 Apr 2025 23:06:33 +0200
From: Michal Luczaj <mhal@...x.co>
To: Stefano Garzarella <sgarzare@...hat.com>,
 Luigi Leonardi <leonardi@...hat.com>
Cc: "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 4/23/25 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?
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?