[PATCHv6 RFC 1/3] nvme-multipath: Add visibility for round-robin io-policy

[Sagi Grimberg]

On 13/12/2024 6:18, Nilay Shroff wrote:
> This patch helps add nvme native multipath visibility for round-robin
> io-policy. It creates a "multipath" sysfs directory under head gendisk
> device node directory and then from "multipath" directory it adds a link
> to each namespace path device the head node refers.
>
> For instance, if we have a shared namespace accessible from two different
> controllers/paths then we create a soft link to each path device from head
> disk node as shown below:
>
> $ ls -l /sys/block/nvme1n1/multipath/
> nvme1c1n1 -> ../../../../../pci052e:78/052e:78:00.0/nvme/nvme1/nvme1c1n1
> nvme1c3n1 -> ../../../../../pci058e:78/058e:78:00.0/nvme/nvme3/nvme1c3n1
>
> In the above example, nvme1n1 is head gendisk node created for a shared
> namespace and the namespace is accessible from nvme1c1n1 and nvme1c3n1
> paths.
>
> For round-robin I/O policy, we could easily infer from the above output
> that I/O workload targeted to nvme1n1 would toggle across paths nvme1c1n1
> and nvme1c3n1.
>
> Signed-off-by: Nilay Shroff <nilay at linux.ibm.com>
> ---
>   drivers/nvme/host/core.c      |  3 ++
>   drivers/nvme/host/multipath.c | 81 +++++++++++++++++++++++++++++++++++
>   drivers/nvme/host/nvme.h      | 18 ++++++--
>   drivers/nvme/host/sysfs.c     | 14 ++++++
>   4 files changed, 112 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index d169a30eb935..df4cc8a27385 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -3982,6 +3982,9 @@ static void nvme_ns_remove(struct nvme_ns *ns)
>   
>   	if (!nvme_ns_head_multipath(ns->head))
>   		nvme_cdev_del(&ns->cdev, &ns->cdev_device);
> +
> +	nvme_mpath_remove_sysfs_link(ns);
> +
>   	del_gendisk(ns->disk);
>   
>   	mutex_lock(&ns->ctrl->namespaces_lock);
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index a85d190942bd..8e2865df2f33 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -686,6 +686,8 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
>   		kblockd_schedule_work(&head->partition_scan_work);
>   	}
>   
> +	nvme_mpath_add_sysfs_link(ns->head);

Why do you add the link in set_live? Why not always set this link? it is 
after all, another
path to the device?