netdev - Re: [PATCH net] sctp: fix an issue that plpmtu can never go to complete state

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Date: Thu, 18 May 2023 12:53:03 -0400
From: Xin Long <lucien.xin@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org, davem@...emloft.net, 
	kuba@...nel.org, Eric Dumazet <edumazet@...gle.com>, 
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net] sctp: fix an issue that plpmtu can never go to
 complete state

On Thu, May 18, 2023 at 6:18 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Tue, 2023-05-16 at 21:22 -0400, Xin Long wrote:
> > When doing plpmtu probe, the probe size is growing every time when it
> > receives the ACK during the Search state until the probe fails. When
> > the failure occurs, pl.probe_high is set and it goes to the Complete
> > state.
> >
> > However, if the link pmtu is huge, like 65535 in loopback_dev, the probe
> > eventually keeps using SCTP_MAX_PLPMTU as the probe size and never fails.
> > Because of that, pl.probe_high can not be set, and the plpmtu probe can
> > never go to the Complete state.
> >
> > Fix it by setting pl.probe_high to SCTP_MAX_PLPMTU when the probe size
> > grows to SCTP_MAX_PLPMTU in sctp_transport_pl_recv(). Also, increase
> > the probe size only when the next is less than SCTP_MAX_PLPMTU.
> >
> > Fixes: b87641aff9e7 ("sctp: do state transition when a probe succeeds on HB ACK recv path")
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  net/sctp/transport.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> >
> > diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> > index 2f66a2006517..b0ccfaa4c1d1 100644
> > --- a/net/sctp/transport.c
> > +++ b/net/sctp/transport.c
> > @@ -324,9 +324,11 @@ bool sctp_transport_pl_recv(struct sctp_transport *t)
> >               t->pl.probe_size += SCTP_PL_BIG_STEP;
> >       } else if (t->pl.state == SCTP_PL_SEARCH) {
> >               if (!t->pl.probe_high) {
> > -                     t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_BIG_STEP,
> > -                                            SCTP_MAX_PLPMTU);
> > -                     return false;
> > +                     if (t->pl.probe_size + SCTP_PL_BIG_STEP < SCTP_MAX_PLPMTU) {
> > +                             t->pl.probe_size += SCTP_PL_BIG_STEP;
> > +                             return false;
> > +                     }
> > +                     t->pl.probe_high = SCTP_MAX_PLPMTU;
>
> It looks like this way the probed mtu can't reach SCTP_MAX_PLPMTU
> anymore, while it was possible before.
indeed.

>
> What about something alike:
>
>                 if (!t->pl.probe_high) {
>                         if (t->pl.probe_size < SCTP_MAX_PLPMTU) {
>                                 t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_BIG_STEP,
>                                                        SCTP_MAX_PLPMTU);
>                                 return false;
>                         }
>                         t->pl.probe_high = SCTP_MAX_PLPMTU;
looks good.

will post v2.

Thanks.

> >               }
> >               t->pl.probe_size += SCTP_PL_MIN_STEP;
> >               if (t->pl.probe_size >= t->pl.probe_high) {
> > @@ -341,7 +343,8 @@ bool sctp_transport_pl_recv(struct sctp_transport *t)
> >       } else if (t->pl.state == SCTP_PL_COMPLETE) {
> >               /* Raise probe_size again after 30 * interval in Search Complete */
> >               t->pl.state = SCTP_PL_SEARCH; /* Search Complete -> Search */
> > -             t->pl.probe_size += SCTP_PL_MIN_STEP;
> > +             if (t->pl.probe_size + SCTP_PL_MIN_STEP < SCTP_MAX_PLPMTU)
> > +                     t->pl.probe_size += SCTP_PL_MIN_STEP;
>
> In a similar way, should the above check be:
>
>                 if (t->pl.probe_size + SCTP_PL_MIN_STEP <= SCTP_MAX_PLPMTU)
>                         t->pl.probe_size += SCTP_PL_MIN_STEP;
>
> or simply:
>                 t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_MIN_STEP, SCTP_MAX_PLPMTU)
> >
> Cheers,
>
> Paolo
>