Re: [RFC PATCH net-next v2 1/2] tcp: Add net.ipv4.tcp_purge_receive_queue sysctl

[Leon Hwang]

On 15/7/26 23:15, Eric Dumazet wrote:
> On Wed, Jul 15, 2026 at 4:54 PM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:

[...]

> 
> My thoughts are:
> 
> out_of_order_queue has been forgotten. skbs could be there and still
> 'block devmem'


Yes. The tp->out_of_order_queue should also be purged here so that
out-of-order SKBs cannot continue holding devmem.

> 
> WRITE_ONCE(tp->copied_seq, tp->rcv_nxt) is certainly wrong, because
> read() will return 0, instead of -1 (errno = EPIPE or ECONNRESET)
> So the application will not know a RST was received :/


Thanks for pointing this out. When sysctl_tcp_purge_receive_queue is
enabled, read() must report the reset error.

The reason read() returns 0 is that SOCK_DONE, which was set when the
FIN was processed, is checked before sk_err in tcp_recvmsg_locked(). I
think clearing SOCK_DONE after purging the queues could let read()
observes the error installed by tcp_done_with_error().

Will also update the packetdrill test to expect -1/EPIPE.

> 
> I think that BSD and linux implementations have historically retained
> acknowledged,
> buffered receive data upon RST to allow applications to drain data
> already ACKed prior to the reset.


Agreed. The new sysctl is disabled by default specifically to preserve
this existing behavior.

Enabling it is an explicit opt-in to discard buffered receive data,
release the associated resources promptly, and report the reset error
immediately.

Will update the change in ip-sysctl.rst documentation with this
application-visible tradeoff.

> 
> Adding a narrow sysctl specifically for CLOSE_WAIT creates
> inconsistent behavior across TCP states.


My bad. After reading the RFC 9293 section 3.10.7.4. [1] again, "All
segment queues should be flushed." should apply to these states:

      -  ESTABLISHED STATE
      -  FIN-WAIT-1 STATE
      -  FIN-WAIT-2 STATE
      -  CLOSE-WAIT STATE

So, the sysctl-controlled purge should be consistently applied to these
four states.

[1] https://www.rfc-editor.org/rfc/rfc9293.html#section-3.10.7.4

Thanks,
Leon