[PATCH 2/3] nvme: add 'queue_if_no_path' semantics

[Christoph Hellwig]

On Mon, Oct 05, 2020 at 11:42:55AM +0200, Hannes Reinecke wrote:
> Instead of reverting the handling of the 'last' path for all
> setups we should be restricting it to the non-fabrics case.
> So add a 'queue_if_no_path' flag which will switch to the
> current behaviour; disabling this flag will revert to the
> original (pre-fabrics) behaviour.
> And set this flag per default for fabrics.
> 
> Signed-off-by: Hannes Reinecke <hare at suse.de>
> ---
>  drivers/nvme/host/core.c      | 7 ++++++-
>  drivers/nvme/host/multipath.c | 4 ++++
>  drivers/nvme/host/nvme.h      | 1 +
>  3 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index af950d58c562..79b88b4c448f 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -474,8 +474,11 @@ static void nvme_free_ns_head(struct kref *ref)
>  	struct nvme_ns_head *head =
>  		container_of(ref, struct nvme_ns_head, ref);
>  
> -	if (head->disk)
> +	if (head->disk) {
> +		if (test_bit(NVME_NSHEAD_QUEUE_IF_NO_PATH, &head->flags))
> +			nvme_mpath_remove_disk(head);
>  		put_disk(head->disk);
> +	}

This needs to be in a helper in multipath.c so that it an compile fine
for the non-multipath case.

> @@ -3686,6 +3689,8 @@ static struct nvme_ns_head *nvme_alloc_ns_head(struct nvme_ctrl *ctrl,
>  	head->subsys = ctrl->subsys;
>  	head->ns_id = nsid;
>  	head->ids = *ids;
> +	if (ctrl->ops->flags & NVME_F_FABRICS)
> +		set_bit(NVME_NSHEAD_QUEUE_IF_NO_PATH, &head->flags);

This needs a comment at very least.  In fact I'm not sure we should
mess with defaults here. 

How will this bit get set for the non-fabrics case?