[dm-devel] [PATCH v2 2/4] nvme: allow local retry for requests with REQ_FAILFAST_TRANSPORT set

[Hannes Reinecke]

On 4/16/21 4:53 PM, Mike Snitzer wrote:
> On Fri, Apr 16 2021 at 10:01am -0400,
> Hannes Reinecke <hare at suse.de> wrote:
> 
>> On 4/16/21 1:15 AM, Mike Snitzer wrote:
>>> From: Chao Leng <lengchao at huawei.com>
>>>
>>> REQ_FAILFAST_TRANSPORT was designed for SCSI, because the SCSI protocol
>>> does not define the local retry mechanism. SCSI implements a fuzzy
>>> local retry mechanism, so REQ_FAILFAST_TRANSPORT is needed to allow
>>> higher-level multipathing software to perform failover/retry.
>>>
>>> NVMe is different with SCSI about this. It defines a local retry
>>> mechanism and path error codes, so NVMe should retry local for non
>>> path error. If path related error, whether to retry and how to retry
>>> is still determined by higher-level multipathing's failover.
>>>
>>> Unlike SCSI, NVMe shouldn't prevent retry if REQ_FAILFAST_TRANSPORT
>>> because NVMe's local retry is needed -- as is NVMe specific logic to
>>> categorize whether an error is path related.
>>>
>>> In this way, the mechanism of NVMe multipath or other multipath are
>>> now equivalent. The mechanism is: non path related error will be
>>> retried locally, path related error is handled by multipath.
>>>
>>> Signed-off-by: Chao Leng <lengchao at huawei.com>
>>> [snitzer: edited header for grammar and clarity, also added code comment]
>>> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
>>> ---
>>>  drivers/nvme/host/core.c | 9 ++++++++-
>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>>> index 540d6fd8ffef..4134cf3c7e48 100644
>>> --- a/drivers/nvme/host/core.c
>>> +++ b/drivers/nvme/host/core.c
>>> @@ -306,7 +306,14 @@ static inline enum nvme_disposition nvme_decide_disposition(struct request *req)
>>>  	if (likely(nvme_req(req)->status == 0))
>>>  		return COMPLETE;
>>>  
>>> -	if (blk_noretry_request(req) ||
>>> +	/*
>>> +	 * REQ_FAILFAST_TRANSPORT is set by upper layer software that
>>> +	 * handles multipathing. Unlike SCSI, NVMe's error handling was
>>> +	 * specifically designed to handle local retry for non-path errors.
>>> +	 * As such, allow NVMe's local retry mechanism to be used for
>>> +	 * requests marked with REQ_FAILFAST_TRANSPORT.
>>> +	 */
>>> +	if ((req->cmd_flags & (REQ_FAILFAST_DEV | REQ_FAILFAST_DRIVER)) ||
>>>  	    (nvme_req(req)->status & NVME_SC_DNR) ||
>>>  	    nvme_req(req)->retries >= nvme_max_retries)
>>>  		return COMPLETE;
>>>
>> Huh?
>>
>> #define blk_noretry_request(rq) \
>>         ((rq)->cmd_flags & (REQ_FAILFAST_DEV|REQ_FAILFAST_TRANSPORT| \
>>                              REQ_FAILFAST_DRIVER))
>>
>> making the only _actual_ change in your patch _not_ evaluating the
>> REQ_FAILFAST_DRIVER, which incidentally is only used by the NVMe core.
> 
> No, not sure how you got there. I'd have thought the 5 references to
> "REQ_FAILFAST_TRANSPORT" would've been sufficient ;)
> 

Ah. Misread stuff. You're excluding the REQ_FAILFAST_TRANSPORT here.
But then it's _actually_ similar to the next patch (which I've also
commented).

Wouldn't it be better to fold them into one patch and discuss things
together; especially as my comment to the next one might actually
achieve the same thing?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		        Kernel Storage Architect
hare at suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)