linux-kernel - Re: [PATCH 2/2] vsock/test: Add test for a linear and non-linear skb getting coalesced

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Message-ID: <0b15644b-9394-4734-9c0e-0a6d1355604a@rbox.co>
Date: Mon, 12 Jan 2026 22:20:50 +0100
From: Michal Luczaj <mhal@...x.co>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: "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>,
 "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>,
 Arseniy Krasnov <avkrasnov@...utedevices.com>, kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] vsock/test: Add test for a linear and non-linear skb
 getting coalesced

On 1/12/26 17:48, Stefano Garzarella wrote:
>>>>>> diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
>>>>>> index bbe3723babdc..21c8616100f1 100644
>>>>>> --- a/tools/testing/vsock/vsock_test.c
>>>>>> +++ b/tools/testing/vsock/vsock_test.c
>>>>>> @@ -2403,6 +2403,11 @@ static struct test_case test_cases[] = {
>>>>>> 		.run_client = test_stream_accepted_setsockopt_client,
>>>>>> 		.run_server = test_stream_accepted_setsockopt_server,
>>>>>> 	},
>>>>>> +	{
>>>>>> +		.name = "SOCK_STREAM MSG_ZEROCOPY coalescence corruption",
>>>>>
>>>>> This is essentially a regression test for virtio transport, so I'd add
>>>>> virtio in the test name.
>>>>
>>>> Isn't virtio transport unaffected? It's about loopback transport (that
>>>> shares common code with virtio transport).
>>>
>>> Why virtio transport is not affected?
>>
>> With the usual caveat that I may be completely missing something, aren't
>> all virtio-transport's rx skbs linear? See virtio_vsock_alloc_linear_skb()
>> in virtio_vsock_rx_fill().
>>
> 
> True, but what about drivers/vhost/vsock.c ?
> 
> IIUC in vhost_vsock_handle_tx_kick() we call vhost_vsock_alloc_skb(), 
> that calls virtio_vsock_alloc_skb() and pass that skb to 
> virtio_transport_recv_pkt(). So, it's also affected right?

virtio_vsock_alloc_skb() returns a non-linear skb only if size >
SKB_WITH_OVERHEAD(PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)). And that is way
more than GOOD_COPY_LEN, so we're good.

At least until someone increases GOOD_COPY_LEN and/or reduces the size
condition for non-linear allocation. So, yeah, a bit brittle.

> BTW in general we consider loopback as one of virtio devices since it 
> really shares with them most of the code.

Fair enough, I'll add "virtio" to the test name.

> That said, now I'm thinking more about Fixes tag.
> Before commit 6693731487a8 ("vsock/virtio: Allocate nonlinear SKBs for 
> handling large transmit buffers") was that a real issue?

I don't really think that commit changes anything for the zerocopy case. It
only makes some big (>GOOD_COPY_LEN) non-ZC skbs turn non-linear.