Re: [PATCH net v2 1/2] vsock/virtio: collapse receive queue under memory pressure

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, Jul 08, 2026 at 07:00:00AM -0400, Michael S. Tsirkin wrote:
>On Wed, Jul 08, 2026 at 12:29:03PM +0200, Stefano Garzarella wrote:
>> From: Stefano Garzarella <sgarzare@redhat.com>
>>
>> When many small packets accumulate in the receive queue, the skb overhead
>> can exceed buf_alloc even while the payload is within bounds. This causes
>> virtio_transport_inc_rx_pkt() to reject packets, leading to connection
>> resets during large transfers under backpressure.
>>
>> The issue was reported by Brien, who has a reproducer, but it is also
>> easily reproducible with iperf-vsock [1] using a small packet size:
>>
>>   iperf3 --vsock -c $CID -l 129
>>
>> which fails immediately without this patch but with commit 059b7dbd20a6
>> ("vsock/virtio: fix potential unbounded skb queue").
>>
>> Inspired by TCP's tcp_collapse() which solves a similar problem, add
>> virtio_transport_collapse_rx_queue() that walks the receive queue and
>> re-copies data into compact linear skbs to reduce the overhead.
>>
>> The collapse is triggered proactively from when the number of skb queued
>> is close to exceeding the overhead budget.
>>
>> A pre-scan counts the eligible bytes to size each allocation precisely,
>> avoiding waste for isolated small packets. Partially consumed skbs are
>> kept as-is to preserve buf_used/fwd_cnt accounting, EOM-marked skbs to
>> maintain SEQPACKET message boundaries, and skbs already larger than the
>> collapse target because they already have a good data-to-overhead ratio.
>>
>> Walking a large queue may take a significant amount of time and cache
>> misses, causing traffic burstiness. To limit this, the collapse stops
>> once enough room is freed for this packet and the next one, but may
>> opportunistically free more to fill each collapsed skb to capacity.
>>
>> [1] https://github.com/stefano-garzarella/iperf-vsock
>>
>> Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
>> Cc: stable@vger.kernel.org
>> Reported-by: Brien Oberstein <brienpub@gmail.com>
>> Closes: https://lore.kernel.org/netdev/618701dd023e$063de350$12b9a9f0$@gmail.com/
>> Tested-by: Brien Oberstein <brienpub@gmail.com>
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>
>
>this is the right approach

Yeah, I have a follow up to start to use skb->truesize, etc. but I guess 
more net-next material.

>
>Acked-by: Michael S. Tsirkin <mst@redhat.com>

Thanks,
Stefano