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

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Date: Wed, 5 Jun 2024 14:01:20 +0800
From: Wen Gu <guwen@...ux.alibaba.com>
To: Gerd Bayer <gbayer@...ux.ibm.com>, wenjia@...ux.ibm.com,
 jaka@...ux.ibm.com, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com
Cc: alibuda@...ux.alibaba.com, tonylu@...ux.alibaba.com,
 linux-s390@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net/smc: avoid overwriting when adjusting sock
 bufsizes



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@linux.alibaba.com
>> Signed-off-by: Wen Gu <guwen@...ux.alibaba.com>
>> ---
>> 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@...ux.ibm.com>, 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