Re: [RFC PATCH net-next] tcp: Add net.ipv4.tcp_purge_receive_queue sysctl

[Leon Hwang]

On 3/3/26 14:26, Leon Hwang wrote:
> 
> 
> On 3/3/26 11:55, Eric Dumazet wrote:
>> On Tue, Mar 3, 2026 at 3:12 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>>>
>>>
>>>
>>> On 3/3/26 08:22, Jakub Kicinski wrote:
>>>> On Mon, 2 Mar 2026 17:55:59 +0800 Leon Hwang wrote:
>>>>> On 26/2/26 09:43, Jakub Kicinski wrote:
>>>>>> On Wed, 25 Feb 2026 15:46:33 +0800 Leon Hwang wrote:
>>>>>>> Issue:
>>>>>>> When a TCP socket in the CLOSE_WAIT state receives a RST packet, the
>>>>>>> current implementation does not clear the socket's receive queue. This
>>>>>>> causes SKBs in the queue to remain allocated until the socket is
>>>>>>> explicitly closed by the application. As a consequence:
>>>>>>>
>>>>>>> 1. The page pool pages held by these SKBs are not released.
>>>>>>
>>>>>> On what kernel version and driver are you observing this?
>>>>>
>>>>> # uname -r
>>>>> 6.19.0-061900-generic
>>>>>
>>>>> # ethtool -i eth0
>>>>> driver: mlx5_core
>>>>> version: 6.19.0-061900-generic
>>>>> firmware-version: 26.43.2566 (MT_0000000531)
>>>>
>>>> Okay... this kernel + driver should just patiently wait for the page
>>>> pool to go away.
>>>>
>>>> What is the actual, end user problem that you're trying to solve?
>>>> A few kB of data waiting to be freed is not a huge problem..
>>>
>>> Yes, it is not a huge problem.
>>>
>>> The actual end-user issue was discussed in
>>> "page_pool: Add page_pool_release_stalled tracepoint" [1].
>>>
>>> I think it would be useful to provide a way for SREs to purge the
>>> receive queue when CLOSE_WAIT TCP sockets receive RST packets. If the
>>> NIC, e.g., Mellanox, flaps, the underlying page pool and pages can be
>>> released at the same time.
>>>
>>> Links:
>>> [1]
>>> https://lore.kernel.org/netdev/b676baa0-2044-4a74-900d-f471620f2896@xxxxxxxxx/
>>
>> Perhaps SRE could use this in an emergency?
>>
>> ss -t -a state close-wait -K
> 
> This ss command is acceptable in an emergency.
> 

However, once a CLOSE_WAIT TCP socket receives an RST packet, it
transitions to the CLOSE state. A socket in the CLOSE state cannot be
killed using the ss approach.

The SKBs remain in the receive queue of the CLOSE socket until it is
closed by the user-space application.

Thanks,
Leon