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From: Mike Snitzer <snitzer@redhat.com>
To: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	dm-devel@redhat.com, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org, Chao Leng <lengchao@huawei.com>
Subject: Re: [PATCH v2 2/4] nvme: allow local retry for requests with REQ_FAILFAST_TRANSPORT set
Date: Fri, 16 Apr 2021 11:32:59 -0400	[thread overview]
Message-ID: <20210416153259.GA20150@redhat.com> (raw)
In-Reply-To: <3c5d6257-5f49-877e-91c2-c6d7687b002b@suse.de>

On Fri, Apr 16 2021 at 11:20am -0400,
Hannes Reinecke <hare@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@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 ;)
> > 
> 
> 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


  reply	other threads:[~2021-04-16 15:33 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-15 23:15 [PATCH v2 0/4] nvme: improve error handling and ana_state to work well with dm-multipath Mike Snitzer
2021-04-15 23:15 ` [PATCH v2 1/4] nvme: return BLK_STS_DO_NOT_RETRY if the DNR bit is set Mike Snitzer
2021-04-15 23:48   ` Chaitanya Kulkarni
2021-04-16 13:51   ` Hannes Reinecke
2021-04-15 23:15 ` [PATCH v2 2/4] nvme: allow local retry for requests with REQ_FAILFAST_TRANSPORT set Mike Snitzer
2021-04-16 14:01   ` Hannes Reinecke
2021-04-16 14:53     ` Mike Snitzer
2021-04-16 15:20       ` Hannes Reinecke
2021-04-16 15:32         ` Mike Snitzer [this message]
2021-04-15 23:15 ` [PATCH v2 3/4] nvme: introduce FAILUP handling for REQ_FAILFAST_TRANSPORT Mike Snitzer
2021-04-16 14:07   ` Hannes Reinecke
2021-04-16 15:03     ` Mike Snitzer
2021-04-16 16:23       ` Hannes Reinecke
2021-04-16 20:03     ` [dm-devel] " Ewan D. Milne
2021-04-16 20:52   ` Ewan D. Milne
2021-04-16 21:44     ` Mike Snitzer
2021-04-15 23:15 ` [PATCH v2 4/4] nvme: decouple basic ANA log page re-read support from native multipathing Mike Snitzer

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