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

[Eric Dumazet]

On Tue, Mar 3, 2026 at 8:55 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>
>
>
> 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.

Why user-space application does not drain the receive queue ?

Is there a missing EPOLLIN or something ?