Re: [PATCH 4/6] nvme-mpath: support controller crd when failing over request

[Hannes Reinecke <hare@suse.de>]

On 8/23/26 10:49 AM, Sagi Grimberg wrote:
> When failing over a request (due to a path based status) we should
> repect controller crd returned in the nvme completion as much as
> possible. Hence we want to delay the failover command execution by
> the controller crdt.
> 
> We allocate a new nvme_mpath_failover_timer referencing the request
> stolen bios in a staging list, and when the command retry delay expires,
> and only then the bios are moved to the mpath head requeue list which is
> immediately kicked to re-submit these bios. If we failed to allocate
> a fot, we fallback to the existing behavior.
> 
> Given that we now have a new staging list for mpath devices, we drain
> them when removing the device.
> 
> Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
> ---
>   drivers/nvme/host/multipath.c | 96 +++++++++++++++++++++++++++++++++--
>   drivers/nvme/host/nvme.h      |  1 +
>   2 files changed, 93 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index b5501217303c..959dd1e05a2d 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -9,6 +9,13 @@
>   #include <trace/events/block.h>
>   #include "nvme.h"
>   
> +struct nvme_mpath_failover_timer {
> +	struct list_head	entry;
> +	struct nvme_ns_head	*head;
> +	struct bio_list		bios;
> +	struct timer_list	timer;
> +};
> +
>   bool multipath = true;
>   static bool multipath_always_on;
>   
> @@ -144,10 +151,48 @@ void nvme_mpath_start_freeze(struct nvme_subsystem *subsys)
>   			blk_freeze_queue_start(h->disk->queue);
>   }
>   
> +static void nvme_mpath_failover_timer_fn(struct timer_list *t)
> +{
> +	struct nvme_mpath_failover_timer *fot = timer_container_of(fot, t, timer);
> +	struct nvme_ns_head *head = fot->head;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&head->requeue_lock, flags);
> +	if (list_empty(&fot->entry)) {
> +		spin_unlock_irqrestore(&head->requeue_lock, flags);
> +		return;
> +	}
> +
> +	list_del_init(&fot->entry);
> +	if (fot->bios.head)
> +		bio_list_merge(&head->requeue_list, &fot->bios);
> +	spin_unlock_irqrestore(&head->requeue_lock, flags);
> +	kblockd_schedule_work(&head->requeue_work);
> +	kfree(fot);
> +}
> +
> +static struct nvme_mpath_failover_timer *
> +nvme_mpath_alloc_failover_timer(struct nvme_ns_head *head)
> +{
> +	struct nvme_mpath_failover_timer *fot;
> +
> +	fot = kzalloc(sizeof(*fot), GFP_ATOMIC);
> +	if (!fot)
> +		goto out;
> +	fot->head = head;
> +	bio_list_init(&fot->bios);
> +	INIT_LIST_HEAD(&fot->entry);
> +	timer_setup(&fot->timer, nvme_mpath_failover_timer_fn, 0);
> +out:
> +	return fot;
> +}
> +
>   void nvme_failover_req(struct request *req)
>   {
>   	struct nvme_ns *ns = req->q->queuedata;
>   	u16 status = nvme_req(req)->status & NVME_SCT_SC_MASK;
> +	struct nvme_mpath_failover_timer *fot = NULL;
> +	unsigned int delay;
>   	unsigned long flags;
>   	struct bio *bio;
>   
> @@ -167,13 +212,27 @@ void nvme_failover_req(struct request *req)
>   	for (bio = req->bio; bio; bio = bio->bi_next)
>   		bio_set_dev(bio, ns->head->disk->part0);
>   
> -	spin_lock_irqsave(&ns->head->requeue_lock, flags);
> -	blk_steal_bios(&ns->head->requeue_list, req);
> -	spin_unlock_irqrestore(&ns->head->requeue_lock, flags);
> +	delay = nvme_crd_msecs(nvme_req(req));
> +	if (delay) {
> +		fot = nvme_mpath_alloc_failover_timer(ns->head);
> +		if (fot) {
> +			blk_steal_bios(&fot->bios, req);
> +			spin_lock_irqsave(&ns->head->requeue_lock, flags);
> +			list_add_tail(&fot->entry, &ns->head->fots);
> +			spin_unlock_irqrestore(&ns->head->requeue_lock, flags);
> +			mod_timer(&fot->timer, jiffies + msecs_to_jiffies(delay));
> +		}
> +	}
> +	/* no CRD or timer allocation failed, fallback to immediate failover */
> +	if (!fot) {
> +		spin_lock_irqsave(&ns->head->requeue_lock, flags);
> +		blk_steal_bios(&ns->head->requeue_list, req);
> +		spin_unlock_irqrestore(&ns->head->requeue_lock, flags);
> +		kblockd_schedule_work(&ns->head->requeue_work);
> +	}

Yikes. Allocation during failover is not going to make you friends.

And this whole mechanism looks pretty similar what we did over at 
implementing CCR. Can you use the mechanism from there?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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