linux-kernel - Re: some questions about restrictions in SMC-R v2's implementation

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Message-ID: <7511f44c-9887-403a-91f3-45f84ff7bb3c@linux.alibaba.com>
Date: Fri, 10 May 2024 19:29:58 +0800
From: Wen Gu <guwen@...ux.alibaba.com>
To: Wenjia Zhang <wenjia@...ux.ibm.com>,
 Guangguan Wang <guangguan.wang@...ux.alibaba.com>, jaka@...ux.ibm.com,
 kgraul@...ux.ibm.com
Cc: linux-s390@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: some questions about restrictions in SMC-R v2's implementation



On 2024/5/10 17:40, Wenjia Zhang wrote:
> 
> 
> On 07.05.24 07:54, Guangguan Wang wrote:
>> Hi, Wenjia and Jan,
>>
>> When testing SMC-R v2, I found some scenarios where SMC-R v2 should be worked, but due to some restrictions in SMC-R 
>> v2's implementation,
>> fallback happened. I want to know why these restrictions exist and what would happen if these restrictions were removed.
>>
>> The first is in the function smc_ib_determine_gid_rcu, where restricts the subnet matching between smcrv2->saddr and 
>> the RDMA related netdev.
>> codes here:
>> static int smc_ib_determine_gid_rcu(...)
>> {
>>      ...
>>          in_dev_for_each_ifa_rcu(ifa, in_dev) {
>>              if (!inet_ifa_match(smcrv2->saddr, ifa))
>>                  continue;
>>              subnet_match = true;
>>              break;
>>          }
>>          if (!subnet_match)
>>              goto out;
>>      ...
>> out:
>>      return -ENODEV;
>> }
>> In my testing environment, either server or client, exists two netdevs, eth0 in netnamespace1 and eth0 in 
>> netnamespace2. For the sake of clarity
>> in the following text, we will refer to eth0 in netnamespace1 as eth1, and eth0 in netnamespace2 as eth2. The eth1's 
>> ip is 192.168.0.3/32 and the
>> eth2's ip is 192.168.0.4/24. The netmask of eth1 must be 32 due to some reasons. The eth1 is a RDMA related netdev, 
>> which means the adaptor of eth1
>> has RDMA function. The eth2 has been associated to the eth1's RDMA device using smc_pnet. When testing connection in 
>> netnamespace2(using eth2 for
>> SMC-R connection), we got fallback connection, rsn is 0x03010000, due to the above subnet matching restriction. But in 
>> this scenario, I think
>> SMC-R should work.
>> In my another testing environment, either server or client, exists two netdevs, eth0 in netnamespace1 and eth1 in 
>> netnamespace1. The eth0's ip is
>> 192.168.0.3/24 and the eth1's ip is 192.168.1.4/24. The eth0 is a RDMA related netdev, which means the adaptor of eth0 
>> has RDMA function. The eth1 has
>> been associated to the eth0's RDMA device using smc_pnet. When testing SMC-R connection through eth1, we got fallback 
>> connection, rsn is 0x03010000,
>> due to the above subnet matching restriction. In my environment, eth0 and eth1 have the same network connectivity even 
>> though they have different
>> subnet. I think SMC-R should work in this scenario.
>>
>> The other is in the function smc_connect_rdma_v2_prepare, where restricts the symmetric configuration of routing 
>> between client and server. codes here:
>> static int smc_connect_rdma_v2_prepare(...)
>> {
>>      ...
>>      if (fce->v2_direct) {
>>          memcpy(ini->smcrv2.nexthop_mac, &aclc->r0.lcl.mac, ETH_ALEN);
>>          ini->smcrv2.uses_gateway = false;
>>      } else {
>>          if (smc_ib_find_route(net, smc->clcsock->sk->sk_rcv_saddr,
>>                smc_ib_gid_to_ipv4(aclc->r0.lcl.gid),
>>                ini->smcrv2.nexthop_mac,
>>                &ini->smcrv2.uses_gateway))
>>              return SMC_CLC_DECL_NOROUTE;
>>          if (!ini->smcrv2.uses_gateway) {
>>              /* mismatch: peer claims indirect, but its direct */
>>              return SMC_CLC_DECL_NOINDIRECT;
>>          }
>>      }
>>      ...
>> }
>> In my testing environment, server's ip is 192.168.0.3/24, client's ip 192.168.0.4/24, regarding how many netdev in 
>> server or client. Server has special
>> route setting due to some other reasons, which results in indirect route from 192.168.0.3/24 to 192.168.0.4/24. Thus, 
>> when CLC handshake, client will
>> get fce->v2_direct==false, but client has no special routing setting and will find direct route from 192.168.0.4/24 to 
>> 192.168.0.3/24. Due to the above
>> symmetric configuration of routing restriction, we got fallback connection, rsn is 0x030f0000. But I think SMC-R 
>> should work in this scenario.
>> And more, why check the symmetric configuration of routing only when server is indirect route?
>>
>> Waiting for your reply.
>>
>> Thanks,
>> Guangguan Wang
>>
> Hi Guangguan,
> 
> Thank you for the questions. We also asked ourselves the same questions a while ago, and also did some research on it. 
> Unfortunately, it was not yet done and I had to delay it because of my vacation last month. Now it's time to pick it up 
> again ;) I'll come back to you as soon as I can give a very certain answer.
> 
> Thanks,
> Wenjia

Hi, Wenjia.

Following Guangguan's questions, I noticed that in SMCv2, ini->smcrv2.saddr stores clcsock->sk->sk_rcv_saddr
and ini->smcrv2.daddr stores the IP converted from peer RNIC's gid (smc_ib_gid_to_ipv4(smc_v2_ext->roce)),
e.g. in smc_find_rdma_v2_device_serv(). And this is also how src address and dst address are considered in many
other places, such as in smc_ib_find_route() mentioned above. I am confused why such 'asymmetrical' usage?

    * clc src addr <----> clc dst addr
    local RNIC gid <----> * peer RNIC gid          (*) means used for saddr or daddr

I guess there might be some reason behind this and I'd really appreciate if you have a answer.

Thank you!