Re: [PATCH v2 4/6] nvme-rdma: avoid IO error and repeated request completion

[Chao Leng <lengchao@huawei.com>]

On 2021/1/21 5:35, Sagi Grimberg wrote:
> 
> is not something we should be handling in nvme. block drivers
>>>>> should be able to fail queue_rq, and this all should live in the
>>>>> block layer.
>>>> Of course, it is also an idea to repair the block drivers directly.
>>>> However, block layer is unaware of nvme native multipathing,
>>>
>>> Nor it should be
>>>
>>>> will cause the request return error which should be avoided.
>>>
>>> Not sure I understand..
>>> requests should failover for path related errors,
>>> what queue_rq errors are expected to be failed over from your
>>> perspective?
>> Although fail over for only path related errors is the best choice, it's
>> almost impossible to achieve.
>> The probability of non-path-related errors is very low. Although these
>> errors do not require fail over retry, the cost of fail over retry
>> is complete the request with error delay a bit long time(retry several
>> times). It's not the best choice, but I think it's acceptable, because
>> HBA driver does not have path-related error codes but only general error
>> codes. It is difficult to identify whether the general error codes are
>> path-related.
> 
> If we have a SW bug or breakage that can happen occasionally, this can
> result in a constant failover rather than a simple failure. This is just
> not a good approach IMO.
> 
>>>> The scenario: use two HBAs for nvme native multipath, and then one HBA
>>>> fault,
>>>
>>> What is the specific error the driver sees?
>> The path related error code is closely related to HBA driver
>> implementation. In general it is EIO. I don't think it's a good idea to
>> assume what general error code the driver returns in the event of a path
>> error.
> 
> But assuming every error is a path error a good idea?
Of course not, according to the old code logic, assuming !ENOMEM && !EAGIAN
for HBA drivers is a path error. I think it might be reasonable.
> 
>>>> the blk_status_t of queue_rq is BLK_STS_IOERR, blk-mq will call
>>>> blk_mq_end_request to complete the request which bypass name native
>>>> multipath. We expect the request fail over to normal HBA, but the request
>>>> is directly completed with BLK_STS_IOERR.
>>>> The two scenarios can be fixed by directly completing the request in queue_rq.
>>> Well, certainly this one-shot always return 0 and complete the command
>>> with HOST_PATH error is not a good approach IMO
>> So what's the better option? Just complete the request with host path
>> error for non-ENOMEM and EAGAIN returned by the HBA driver?
> 
> Well, the correct thing to do here would be to clone the bio and
> failover if the end_io error status is BLK_STS_IOERR. That sucks
> because it adds overhead, but this proposal doesn't sit well. it
> looks wrong to me.
> 
> Alternatively, a more creative idea would be to encode the error
> status somehow in the cookie returned from submit_bio, but that
> also feels like a small(er) hack.
If HBA drivers return !ENOMEM && !EAGIAN, queue_rq Directly call
nvme_complete_rq with NVME_SC_HOST_PATH_ERROR like
nvmf_fail_nonready_command. nvme_complete_rq will decide to retry,
fail over or end the request. This may not be the best, but there seems
to be no better choice.
I will try to send the patch v2.