Re: [PATCH v2 net-next 06/21] nvme-tcp: Add DDP offload control path

[Boris Pismenny <borispismenny@gmail.com>]

On 19/01/2021 5:47, David Ahern wrote:
> On 1/14/21 8:10 AM, Boris Pismenny wrote:
>> +static
>> +int nvme_tcp_offload_socket(struct nvme_tcp_queue *queue)
>> +{
>> +	struct net_device *netdev = get_netdev_for_sock(queue->sock->sk, true);
>> +	struct nvme_tcp_ddp_config config = {};
>> +	int ret;
>> +
>> +	if (!netdev) {
>> +		dev_info_ratelimited(queue->ctrl->ctrl.device, "netdev not found\n");
>> +		return -ENODEV;
>> +	}
>> +
>> +	if (!(netdev->features & NETIF_F_HW_TCP_DDP)) {
>> +		dev_put(netdev);
>> +		return -EOPNOTSUPP;
>> +	}
>> +
>> +	config.cfg.type		= TCP_DDP_NVME;
>> +	config.pfv		= NVME_TCP_PFV_1_0;
>> +	config.cpda		= 0;
>> +	config.dgst		= queue->hdr_digest ?
>> +		NVME_TCP_HDR_DIGEST_ENABLE : 0;
>> +	config.dgst		|= queue->data_digest ?
>> +		NVME_TCP_DATA_DIGEST_ENABLE : 0;
>> +	config.queue_size	= queue->queue_size;
>> +	config.queue_id		= nvme_tcp_queue_id(queue);
>> +	config.io_cpu		= queue->io_cpu;
>> +
>> +	ret = netdev->tcp_ddp_ops->tcp_ddp_sk_add(netdev,
>> +						  queue->sock->sk,
>> +						  (struct tcp_ddp_config *)&config);
> 
> typecast is not needed; tcp_ddp_config is an element of nvme_tcp_ddp_config
> 

True, will fix, thanks!

>> +	if (ret) {
>> +		dev_put(netdev);
>> +		return ret;
>> +	}
>> +
>> +	inet_csk(queue->sock->sk)->icsk_ulp_ddp_ops = &nvme_tcp_ddp_ulp_ops;
>> +	if (netdev->features & NETIF_F_HW_TCP_DDP)
>> +		set_bit(NVME_TCP_Q_OFF_DDP, &queue->flags);
>> +
>> +	return ret;
>> +}
>> +
>> +static
>> +void nvme_tcp_unoffload_socket(struct nvme_tcp_queue *queue)
>> +{
>> +	struct net_device *netdev = queue->ctrl->offloading_netdev;
>> +
>> +	if (!netdev) {
>> +		dev_info_ratelimited(queue->ctrl->ctrl.device, "netdev not found\n");
>> +		return;
>> +	}
>> +
>> +	netdev->tcp_ddp_ops->tcp_ddp_sk_del(netdev, queue->sock->sk);
>> +
>> +	inet_csk(queue->sock->sk)->icsk_ulp_ddp_ops = NULL;
>> +	dev_put(netdev); /* put the queue_init get_netdev_for_sock() */
> 
> have you validated the netdev reference counts? You have a put here, and ...
> 

Yes, it does work for the cases we've tested: up/down,
connect/disconnect, and up/down during traffic. It is
unfortunate that it is not trivial to follow.
We'll add some comment to make it more clear. Also see
below.

>> +}
>> +
>> +static
>> +int nvme_tcp_offload_limits(struct nvme_tcp_queue *queue)
>> +{
>> +	struct net_device *netdev = get_netdev_for_sock(queue->sock->sk, true);
> 
> ... a get here ....
> 
>> +	struct tcp_ddp_limits limits;
>> +	int ret = 0;
>> +
>> +	if (!netdev) {
>> +		dev_info_ratelimited(queue->ctrl->ctrl.device, "netdev not found\n");
>> +		return -ENODEV;
>> +	}
>> +
>> +	if (netdev->features & NETIF_F_HW_TCP_DDP &&
>> +	    netdev->tcp_ddp_ops &&
>> +	    netdev->tcp_ddp_ops->tcp_ddp_limits)
>> +		ret = netdev->tcp_ddp_ops->tcp_ddp_limits(netdev, &limits);
>> +	else
>> +		ret = -EOPNOTSUPP;
>> +
>> +	if (!ret) {
>> +		queue->ctrl->offloading_netdev = netdev;
> 
> 
> ... you have the device here, but then ...
> 
>> +		dev_dbg_ratelimited(queue->ctrl->ctrl.device,
>> +				    "netdev %s offload limits: max_ddp_sgl_len %d\n",
>> +				    netdev->name, limits.max_ddp_sgl_len);
>> +		queue->ctrl->ctrl.max_segments = limits.max_ddp_sgl_len;
>> +		queue->ctrl->ctrl.max_hw_sectors =
>> +			limits.max_ddp_sgl_len << (ilog2(SZ_4K) - 9);
>> +	} else {
>> +		queue->ctrl->offloading_netdev = NULL;
>> +	}
>> +
>> +	dev_put(netdev);
> 
> ... put here. And this is the limit checking function which seems like
> an odd place to set offloading_netdev vs nvme_tcp_offload_socket which
> sets no queue variable but yet hangs on to a netdev reference count.
> 
> netdev reference count leaks are an absolute PITA to find. Code that
> takes and puts the counts should be clear and obvious as to when and
> why. The symmetry of offload and unoffload are clear when the offload
> saves the address in offloading_netdev. What you have now is dubious.
> 

The idea here is to rely on offload and unoffload to hold the netdev
during offload. Get limits is not offloading anything; it only queries
device limits that are then applied to the queue by the caller.
We hold the device here for only to ensure that the function is still
there when it is called, and we release it once we are done with it,
as no context is established on the NIC, and no offload takes place.