Re: [PATCH v3 net-next 1/3] ipv6: remove dst_allfrag test on ipv6 output

[Eric Dumazet]

On Fri, Oct 20, 2023 at 8:39 AM Yan Zhai <yan@xxxxxxxxxxxxxx> wrote:
>
> On Fri, Oct 20, 2023 at 1:06 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Fri, Oct 20, 2023 at 7:32 AM Yan Zhai <yan@xxxxxxxxxxxxxx> wrote:
> > >
> > > dst_allfrag was added before the first git commit:
> > >
> > > https://www.mail-archive.com/bk-commits-head@xxxxxxxxxxxxxxx/msg03399.html
> > >
> > > The feature would send packets to the fragmentation path if a box
> > > receives a PMTU value with less than 1280 byte. However, since commit
> > > 9d289715eb5c ("ipv6: stop sending PTB packets for MTU < 1280"), such
> > > message would be simply discarded. The feature flag is neither supported
> > > in iproute2 utility. In theory one can still manipulate it with direct
> > > netlink message, but it is not ideal because it was based on obsoleted
> > > guidance of RFC-2460 (replaced by RFC-8200).
> > >
> > > The feature test would always return false at the moment, so remove it
> > > from the output path.
> >
> > What about other callers of dst_allfrag() ?
> >
> > This feature seems broken atm.
>
> It is broken as far as I can tell. The reason I removed just one
> caller here is to keep the code simpler and consistent. If I don't do
> so, I ought to test it for both GSO fast path and slow path to be
> logically consistent. Seems an overkill to me. For the removal of the
> rest, I'd hope it could come in as a standalone patch(set) because it
> is not just callers but also those unnecessary flags and tests on IP
> corks and sockets, not quite aligned with this patch's intention. I
> noted you have drafted something like this in the past:
>
> https://lkml.kernel.org/netdev/1335348157.3274.30.camel@edumazet-glaptop/
>
> I guess it might be a good base point to work on as a new patch(set)?
> What's your call on this?
>

I am about to send a TCP patch series to enable usec TSval (instead of ms),
and was planning to use a new RTAX_FEATURE_TCP_USEC_TS.

I also noticed that iproute2 was not supporting RTAX_FEATURE_ALLFRAG,
so we might kill it completely ?

commit b258c87639f77d772c077a4e45dad602c62c9f1c
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Wed Oct 18 09:33:38 2023 +0000

    tcp: add RTAX_FEATURE_TCP_USEC_TS

    This new dst feature flag will be used to allow TCP to use usec
    based timestamps instead of msec ones.

    ip route .... feature tcp_usec_ts

    Also document that RTAX_FEATURE_SACK and RTAX_FEATURE_TIMESTAMP
    are unused.

    Note that iproute2 does yet support RTAX_FEATURE_ALLFRAG ?

    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>

diff --git a/include/uapi/linux/rtnetlink.h b/include/uapi/linux/rtnetlink.h
index 51c13cf9c5aee4a2d1ab33c1a89043383d67b9cf..aa2482a0614aa685590fcc73819cbe1baac63d66
100644
--- a/include/uapi/linux/rtnetlink.h
+++ b/include/uapi/linux/rtnetlink.h
@@ -502,13 +502,17 @@ enum {

 #define RTAX_MAX (__RTAX_MAX - 1)

-#define RTAX_FEATURE_ECN       (1 << 0)
-#define RTAX_FEATURE_SACK      (1 << 1)
-#define RTAX_FEATURE_TIMESTAMP (1 << 2)
-#define RTAX_FEATURE_ALLFRAG   (1 << 3)
-
-#define RTAX_FEATURE_MASK      (RTAX_FEATURE_ECN | RTAX_FEATURE_SACK | \
-                                RTAX_FEATURE_TIMESTAMP | RTAX_FEATURE_ALLFRAG)
+#define RTAX_FEATURE_ECN               (1 << 0)
+#define RTAX_FEATURE_SACK              (1 << 1) /* unused */
+#define RTAX_FEATURE_TIMESTAMP         (1 << 2) /* unused */
+#define RTAX_FEATURE_ALLFRAG           (1 << 3)
+#define RTAX_FEATURE_TCP_USEC_TS       (1 << 4)
+
+#define RTAX_FEATURE_MASK      (RTAX_FEATURE_ECN |             \
+                                RTAX_FEATURE_SACK |            \
+                                RTAX_FEATURE_TIMESTAMP |       \
+                                RTAX_FEATURE_ALLFRAG |         \
+                                RTAX_FEATURE_TCP_USEC_TS)

 struct rta_session {
        __u8    proto;