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

[Mike Snitzer]

On Fri, Apr 16 2021 at 11:20am -0400,
Hannes Reinecke <hare at suse.de> wrote:

> 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?

2 discrete things. This patch enables local retry.
Patch 3 allows proper failover via upper layer multipathing.

And as I replied, your suggestion about using DNR doesn't achieve the
same thing (said as much in reply to the patch 3 thread).

Mike