Re: [PATCH net] net/smc: avoid overwriting when adjusting sock bufsizes

[Wen Gu]

On 2024/6/5 00:16, Gerd Bayer wrote:
> Hi Wen Gu,
>
> sorry for the late reply, I'm just catching up after a bit of a
> vacation.

No worries at all, I hope you had a great vacation!

> On Fri, 2024-05-31 at 16:54 +0800, Wen Gu wrote:
> > When copying smc settings to clcsock, avoid setting clcsock's
> > sk_sndbuf to sysctl_tcp_wmem[1], since this may overwrite the value
> > set by tcp_sndbuf_expand() in TCP connection establishment.
> >
> > And the other setting sk_{snd|rcv}buf to sysctl value in
> > smc_adjust_sock_bufsizes() can also be omitted since the
> > initialization of smc sock and clcsock has set sk_{snd|rcv}buf to
> > smc.sysctl_{w|r}mem or ipv4_sysctl_tcp_{w|r}mem[1].
> >
> > Fixes: 30c3c4a4497c ("net/smc: Use correct buffer sizes when
> > switching between TCP and SMC")
> > Link:
> > https://lore.kernel.org/r/5eaf3858-e7fd-4db8-83e8-3d7a3e0e9ae2@xxxxxxxxxxxxxxxxx
> > Signed-off-by: Wen Gu <guwen@xxxxxxxxxxxxxxxxx>
> > ---
> > FYI,
> > The detailed motivation and testing can be found in the link above.
> > ---

<...>

> As Wenjia already said, we've discussed this a bit.
> As I remember, I've added the sections to copy over the sysctl values
> as a "safety measure" when moving between smc/clc sockets - but had the
> wrong assumption in mind that e.g. in a fall-back a new TCP handshake
> would be done. Apparently, we didn't test the buffer size behavior in
> these scenarios enough to notice the "weird" behavior.
>
> So we reviewed your initial report of the oddity per your message in
> the link above, too.
>
> We fully agree that if no connection at the SMC level could be
> established, you should expect the socket buffersizes be used that had
> been established for the TCP connection - regardless if the fallback is
> due to the server or the client.
>
> So feel free to add my
> Reviewed-by: Gerd Bayer <gbayer@xxxxxxxxxxxxx>, too.

Hi, Gerd and Wenjia. Thanks a lot for your confirmation.

And as for the last question in the initial report (link above), that
why the server does not call smc_copy_sock_settings_to_clc() like the
client when fallback happens, I guess it is because at the time that
server fallback, the new_smc sock has not been accepted, so there will
be no user's sock settings that needs to be copied to clcsock.

Thanks!

> Thanks,
> Gerd