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

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Apr 16 2021 at 10:01am -0400,
Hannes Reinecke <hare@suse.de> wrote:

> On 4/16/21 1:15 AM, Mike Snitzer wrote:
> > From: Chao Leng <lengchao@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@huawei.com>
> > [snitzer: edited header for grammar and clarity, also added code comment]
> > Signed-off-by: Mike Snitzer <snitzer@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 ;)

This patch makes it so requests marked with REQ_FAILFAST_TRANSPORT are
allowed to use NVMe's local retry (that is required for non-transport
errors).

> So what is it you're trying to solve?

What the patch header, code and code comment detail.

Mike

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