Re: nvme: explicitly use normal NVMe error handling when appropriate

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Aug 10 2020 at 11:32pm -0400,
Chao Leng <lengchao@huawei.com> wrote:

> 
> 
> On 2020/8/11 1:22, Mike Snitzer wrote:
> >On Mon, Aug 10 2020 at 10:36am -0400,
> >Mike Snitzer <snitzer@redhat.com> wrote:
> >
> >>On Fri, Aug 07 2020 at  7:35pm -0400,
> >>Sagi Grimberg <sagi@grimberg.me> wrote:
> >>
> >>>
> >>>>>Hey Mike,
> >...
> >>>>I think NVMe can easily fix this by having an earlier stage of checking,
> >>>>e.g. nvme_local_retry_req(), that shortcircuits ever getting to
> >>>>higher-level multipathing consideration (be it native NVMe or DM
> >>>>multipathing) for cases like NVME_SC_CMD_INTERRUPTED.
> >>>>To be clear: the "default" case of nvme_failover_req() that returns
> >>>>false to fallback to NVMe's "local" normal NVMe error handling -- that
> >>>>can stay.. but a more explicit handling of cases like
> >>>>NVME_SC_CMD_INTERRUPTED should be added to a nvme_local_retry_req()
> >>>>check that happens before nvme_failover_req() in nvme_complete_rq().
> >>>
> >>>I don't necessarily agree with having a dedicated nvme_local_retry_req().
> >>>a request that isn't failed over, goes to local error handling (retry or
> >>>not). I actually think that just adding the condition to
> >>>nvme_complete_req and having nvme_failover_req reject it would work.
> >>>
> >>>Keith?
> >>
> >>I think that is basically what I'm thinking too.
> >
> >From: Mike Snitzer <snitzer@redhat.com>
> >Subject: nvme: explicitly use normal NVMe error handling when appropriate
> >
> >Commit 764e9332098c0 ("nvme-multipath: do not reset on unknown
> >status"), among other things, fixed NVME_SC_CMD_INTERRUPTED error
> >handling by changing multipathing's nvme_failover_req() to short-circuit
> >path failover and then fallback to NVMe's normal error handling (which
> >takes care of NVME_SC_CMD_INTERRUPTED).
> >
> >This detour through native NVMe multipathing code is unwelcome because
> >it prevents NVMe core from handling NVME_SC_CMD_INTERRUPTED independent
> >of any multipathing concerns.
> >
> >Introduce nvme_status_needs_local_error_handling() to prioritize
> >non-failover retry, when appropriate, in terms of normal NVMe error
> >handling.  nvme_status_needs_local_error_handling() will naturely evolve
> >to include handling of any other errors that normal error handling must
> >be used for.
> >
> >nvme_failover_req()'s ability to fallback to normal NVMe error handling
> >has been preserved because it may be useful for future NVME_SC that
> >nvme_status_needs_local_error_handling() hasn't yet been trained for.
> >
> >Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> >---
> >  drivers/nvme/host/core.c | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> >
> >diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> >index 88cff309d8e4..be749b690af7 100644
> >--- a/drivers/nvme/host/core.c
> >+++ b/drivers/nvme/host/core.c
> >@@ -252,6 +252,16 @@ static inline bool nvme_req_needs_retry(struct request *req)
> >  	return true;
> >  }
> >+static inline bool nvme_status_needs_local_error_handling(u16 status)
> >+{
> >+	switch (status & 0x7ff) {
> >+	case NVME_SC_CMD_INTERRUPTED:
> >+		return true;
> >+	default:
> >+		return false;
> >+	}
> >+}
> >+
> >  static void nvme_retry_req(struct request *req)
> >  {
> >  	struct nvme_ns *ns = req->q->queuedata;
> >@@ -270,7 +280,8 @@ static void nvme_retry_req(struct request *req)
> >  void nvme_complete_rq(struct request *req)
> >  {
> >-	blk_status_t status = nvme_error_status(nvme_req(req)->status);
> >+	u16 nvme_status = nvme_req(req)->status;
> >+	blk_status_t status = nvme_error_status(nvme_status);
> >  	trace_nvme_complete_rq(req);
> >@@ -280,7 +291,8 @@ void nvme_complete_rq(struct request *req)
> >  		nvme_req(req)->ctrl->comp_seen = true;
> >  	if (unlikely(status != BLK_STS_OK && nvme_req_needs_retry(req))) {
> >-		if ((req->cmd_flags & REQ_NVME_MPATH) && nvme_failover_req(req))
> >+		if (!nvme_status_needs_local_error_handling(nvme_status) &&
> >+		    (req->cmd_flags & REQ_NVME_MPATH) && nvme_failover_req(req))
>
> This looks no affect. if work with nvme multipath, now is already retry local.

Not if NVMe is built without multipathing configured.

> If work with dm-multipath, still return error.

Yes, I'm aware.  Use of REQ_FAILFAST_TRANSPORT isn't something that is
needed for NVMe, so why are you proposing hacks in NVMe to deal with it?

> >  			return;
> >  		if (!blk_queue_dying(req->q)) {
> >
> 
> Suggest:
> REQ_FAILFAST_TRANSPORT may be designed for scsi, because scsi protocol
> do not difine the local retry mechanism. SCSI implements a fuzzy local
> retry mechanism, so need the REQ_FAILFAST_TRANSPORT for multipath
> software, multipath software retry according error code is expected.
> nvme is different with scsi about this. It define local retry mechanism
> and path error code, so nvme should not care REQ_FAILFAST_TRANSPORT.

Exactly.  Except by "nvme should not care REQ_FAILFAST_TRANSPORT." your
patch says you mean "nvme shouldn't disallow retry if
REQ_FAILFAST_TRANSPORT is it".  I'm saying: don't try to get such
changes into NVMe.

In general, aspects of your patch may have merit but overall it is doing
too much.

Mike


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