Re: [PATCH 02/19] nvme: introduce a namespace count in the ns head structure

[Nilay Shroff <nilay@linux.ibm.com>]

On 2/25/26 9:09 PM, John Garry wrote:
> For switching to use libmultipath, the per-namespace sibling list entry in
> nvme_ns.sibling will be replaced with multipath_device.sibling list
> pointer.
> 
> For when CONFIG_LIBMULTIPATH is disabled, that list of namespaces would no
> longer be maintained.
> 
> However the core code checks in many places whether there is any
> namespace in the head list, like in nvme_ns_remove().
> 
> Introduce a separate count of the number of namespaces for the namespace
> head and use that count for the places where the per-namespace head list
> of namespaces is checked to be empty.
> 
> Signed-off-by: John Garry<john.g.garry@oracle.com>
> ---
>   drivers/nvme/host/core.c      | 10 +++++++---
>   drivers/nvme/host/multipath.c |  4 ++--
>   drivers/nvme/host/nvme.h      |  1 +
>   3 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 37e30caff4149..76249871dd7c2 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -4024,7 +4024,7 @@ static int nvme_init_ns_head(struct nvme_ns *ns, struct nvme_ns_info *info)
>   	} else {
>   		ret = -EINVAL;
>   		if ((!info->is_shared || !head->shared) &&
> -		    !list_empty(&head->list)) {
> +		    head->ns_count) {
>   			dev_err(ctrl->device,
>   				"Duplicate unshared namespace %d\n",
>   				info->nsid);
> @@ -4047,6 +4047,7 @@ static int nvme_init_ns_head(struct nvme_ns *ns, struct nvme_ns_info *info)
>   	}
>   
>   	list_add_tail_rcu(&ns->siblings, &head->list);
> +	head->ns_count++;
>   	ns->head = head;
>   	mutex_unlock(&ctrl->subsys->lock);
>   

I think we could still access head->mpath_disk->mpath_head->dev_list.
So in that case do we really need to have ->ns_count? Moreover, if
we could maintain a pointer to struct mpath_head from struct 
nvme_ns_head then we may avoid one dereference. What do you think?

Thanks,
--Nilay