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Subject: Re: [PATCH v8 13/13] nvmet: Optionally use PCI P2P memory
To:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-block@vger.kernel.org
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Keith Busch <keith.busch@intel.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Jens Axboe <axboe@kernel.dk>,
        Steve Wise <swise@opengridcomputing.com>
References: <20180927165420.5290-1-logang@deltatee.com>
 <20180927165420.5290-14-logang@deltatee.com>
From:   Sagi Grimberg <sagi@grimberg.me>
Message-ID: <5ddeed51-0580-9581-cf12-c75e18b4f7cc@grimberg.me>
Date:   Mon, 1 Oct 2018 14:34:20 -0700
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On 09/27/2018 09:54 AM, Logan Gunthorpe wrote:
> We create a configfs attribute in each nvme-fabrics target port to
> enable p2p memory use. When enabled, the port will only then use the
> p2p memory if a p2p memory device can be found which is behind the
> same switch hierarchy as the RDMA port and all the block devices in
> use. If the user enabled it and no devices are found, then the system
> will silently fall back on using regular memory.
> 
> If appropriate, that port will allocate memory for the RDMA buffers
> for queues from the p2pmem device falling back to system memory should
> anything fail.
> 
> Ideally, we'd want to use an NVME CMB buffer as p2p memory. This would
> save an extra PCI transfer as the NVME card could just take the data
> out of it's own memory. However, at this time, only a limited number
> of cards with CMB buffers seem to be available.
> 
> Signed-off-by: Stephen Bates <sbates@raithlin.com>
> Signed-off-by: Steve Wise <swise@opengridcomputing.com>
> [hch: partial rewrite of the initial code]
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>   drivers/nvme/target/configfs.c    |  36 ++++++++
>   drivers/nvme/target/core.c        | 138 +++++++++++++++++++++++++++++-
>   drivers/nvme/target/io-cmd-bdev.c |   3 +
>   drivers/nvme/target/nvmet.h       |  13 +++
>   drivers/nvme/target/rdma.c        |   2 +
>   5 files changed, 191 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/target/configfs.c b/drivers/nvme/target/configfs.c
> index b37a8e3e3f80..0dfb0e0c3d21 100644
> --- a/drivers/nvme/target/configfs.c
> +++ b/drivers/nvme/target/configfs.c
> @@ -17,6 +17,8 @@
>   #include <linux/slab.h>
>   #include <linux/stat.h>
>   #include <linux/ctype.h>
> +#include <linux/pci.h>
> +#include <linux/pci-p2pdma.h>
>   
>   #include "nvmet.h"
>   
> @@ -1094,6 +1096,37 @@ static void nvmet_port_release(struct config_item *item)
>   	kfree(port);
>   }
>   
> +#ifdef CONFIG_PCI_P2PDMA
> +static ssize_t nvmet_p2pmem_show(struct config_item *item, char *page)
> +{
> +	struct nvmet_port *port = to_nvmet_port(item);
> +
> +	return pci_p2pdma_enable_show(page, port->p2p_dev, port->use_p2pmem);
> +}
> +
> +static ssize_t nvmet_p2pmem_store(struct config_item *item,
> +		const char *page, size_t count)
> +{
> +	struct nvmet_port *port = to_nvmet_port(item);
> +	struct pci_dev *p2p_dev = NULL;
> +	bool use_p2pmem;
> +	int error;
> +
> +	error = pci_p2pdma_enable_store(page, &p2p_dev, &use_p2pmem);
> +	if (error)
> +		return error;
> +
> +	down_write(&nvmet_config_sem);
> +	port->use_p2pmem = use_p2pmem;
> +	pci_dev_put(port->p2p_dev);
> +	port->p2p_dev = p2p_dev;
> +	up_write(&nvmet_config_sem);
> +
> +	return count;
> +}
> +CONFIGFS_ATTR(nvmet_, p2pmem);
> +#endif /* CONFIG_PCI_P2PDMA */
> +
>   static struct configfs_attribute *nvmet_port_attrs[] = {
>   	&nvmet_attr_addr_adrfam,
>   	&nvmet_attr_addr_treq,
> @@ -1101,6 +1134,9 @@ static struct configfs_attribute *nvmet_port_attrs[] = {
>   	&nvmet_attr_addr_trsvcid,
>   	&nvmet_attr_addr_trtype,
>   	&nvmet_attr_param_inline_data_size,
> +#ifdef CONFIG_PCI_P2PDMA
> +	&nvmet_attr_p2pmem,
> +#endif
>   	NULL,
>   };
>   
> diff --git a/drivers/nvme/target/core.c b/drivers/nvme/target/core.c
> index bddd1599b826..7ade16cb4ed3 100644
> --- a/drivers/nvme/target/core.c
> +++ b/drivers/nvme/target/core.c
> @@ -15,6 +15,7 @@
>   #include <linux/module.h>
>   #include <linux/random.h>
>   #include <linux/rculist.h>
> +#include <linux/pci-p2pdma.h>
>   
>   #include "nvmet.h"
>   
> @@ -365,9 +366,29 @@ static void nvmet_ns_dev_disable(struct nvmet_ns *ns)
>   	nvmet_file_ns_disable(ns);
>   }
>   
> +static int nvmet_p2pdma_add_client(struct nvmet_ctrl *ctrl,
> +		struct nvmet_ns *ns)
> +{
> +	int ret;
> +
> +	if (!blk_queue_pci_p2pdma(ns->bdev->bd_queue)) {
> +		pr_err("peer-to-peer DMA is not supported by %s\n",
> +		       ns->device_path);
> +		return -EINVAL;
> +	}
> +
> +	ret = pci_p2pdma_add_client(&ctrl->p2p_clients, nvmet_ns_dev(ns));
> +	if (ret)
> +		pr_err("failed to add peer-to-peer DMA client %s: %d\n",
> +		       ns->device_path, ret);
> +
> +	return ret;
> +}
> +
>   int nvmet_ns_enable(struct nvmet_ns *ns)
>   {
>   	struct nvmet_subsys *subsys = ns->subsys;
> +	struct nvmet_ctrl *ctrl;
>   	int ret;
>   
>   	mutex_lock(&subsys->lock);
> @@ -389,6 +410,14 @@ int nvmet_ns_enable(struct nvmet_ns *ns)
>   	if (ret)
>   		goto out_dev_put;
>   
> +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
> +		if (ctrl->p2p_dev) {
> +			ret = nvmet_p2pdma_add_client(ctrl, ns);
> +			if (ret)
> +				goto out_remove_clients;
> +		}
> +	}
> +
>   	if (ns->nsid > subsys->max_nsid)
>   		subsys->max_nsid = ns->nsid;
>   
> @@ -417,6 +446,9 @@ int nvmet_ns_enable(struct nvmet_ns *ns)
>   out_unlock:
>   	mutex_unlock(&subsys->lock);
>   	return ret;
> +out_remove_clients:
> +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
> +		pci_p2pdma_remove_client(&ctrl->p2p_clients, nvmet_ns_dev(ns));
>   out_dev_put:
>   	nvmet_ns_dev_disable(ns);
>   	goto out_unlock;
> @@ -425,6 +457,7 @@ int nvmet_ns_enable(struct nvmet_ns *ns)
>   void nvmet_ns_disable(struct nvmet_ns *ns)
>   {
>   	struct nvmet_subsys *subsys = ns->subsys;
> +	struct nvmet_ctrl *ctrl;
>   
>   	mutex_lock(&subsys->lock);
>   	if (!ns->enabled)
> @@ -450,6 +483,12 @@ void nvmet_ns_disable(struct nvmet_ns *ns)
>   	percpu_ref_exit(&ns->ref);
>   
>   	mutex_lock(&subsys->lock);
> +
> +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
> +		pci_p2pdma_remove_client(&ctrl->p2p_clients, nvmet_ns_dev(ns));
> +		nvmet_add_async_event(ctrl, NVME_AER_TYPE_NOTICE, 0, 0);

Hi Logan, what is this event here?

> +	}
> +
>   	subsys->nr_namespaces--;
>   	nvmet_ns_changed(subsys, ns->nsid);
>   	nvmet_ns_dev_disable(ns);
> @@ -727,6 +766,23 @@ EXPORT_SYMBOL_GPL(nvmet_req_execute);
>   
>   int nvmet_req_alloc_sgl(struct nvmet_req *req, struct nvmet_sq *sq)
>   {
> +	struct pci_dev *p2p_dev = NULL;
> +
> +	if (IS_ENABLED(CONFIG_PCI_P2PDMA)) {
> +		if (sq->ctrl)
> +			p2p_dev = sq->ctrl->p2p_dev;
> +
> +		req->p2p_dev = NULL;
> +		if (sq->qid && p2p_dev) {
> +			req->sg = pci_p2pmem_alloc_sgl(p2p_dev, &req->sg_cnt,
> +						       req->transfer_len);
> +			if (req->sg) {
> +				req->p2p_dev = p2p_dev;
> +				return 0;
> +			}

Would be useful to comment that we fall to normal sgl allocation.

> +		}
> +	}
> +
>   	req->sg = sgl_alloc(req->transfer_len, GFP_KERNEL, &req->sg_cnt);
>   	if (!req->sg)
>   		return -ENOMEM;
> @@ -737,7 +793,11 @@ EXPORT_SYMBOL_GPL(nvmet_req_alloc_sgl);
>   
>   void nvmet_req_free_sgl(struct nvmet_req *req)
>   {
> -	sgl_free(req->sg);
> +	if (req->p2p_dev)
> +		pci_p2pmem_free_sgl(req->p2p_dev, req->sg);
> +	else
> +		sgl_free(req->sg);
> +
>   	req->sg = NULL;
>   	req->sg_cnt = 0;
>   }
> @@ -939,6 +999,79 @@ bool nvmet_host_allowed(struct nvmet_req *req, struct nvmet_subsys *subsys,
>   		return __nvmet_host_allowed(subsys, hostnqn);
>   }
>   
> +/*
> + * If allow_p2pmem is set, we will try to use P2P memory for the SGL lists for
> + * Ι/O commands. This requires the PCI p2p device to be compatible with the
> + * backing device for every namespace on this controller.
> + */
> +static void nvmet_setup_p2pmem(struct nvmet_ctrl *ctrl, struct nvmet_req *req)
> +{
> +	struct nvmet_ns *ns;
> +	int ret;
> +
> +	if (!req->port->use_p2pmem || !req->p2p_client)
> +		return;

Nit, IMO would be better to check at the call-site, but not a hard
must...

I still do not fully understand why p2p_dev has to be ctrl-wide and not
per namespace. Sorry to keep bringing this up (again). But if people are
OK with it then I guess I can stop asking about this...


> +
> +	mutex_lock(&ctrl->subsys->lock);
> +
> +	ret = pci_p2pdma_add_client(&ctrl->p2p_clients, req->p2p_client);
> +	if (ret) {
> +		pr_err("failed adding peer-to-peer DMA client %s: %d\n",
> +		       dev_name(req->p2p_client), ret);
> +		goto free_devices;
> +	}
> +
> +	list_for_each_entry_rcu(ns, &ctrl->subsys->namespaces, dev_link) {
> +		ret = nvmet_p2pdma_add_client(ctrl, ns);
> +		if (ret)
> +			goto free_devices;

I think that at some point we said that this looks like it should fall
back to host memory for those namespaces.. when we allocate the sgl we
already assigned a namespace to the request (nvmet_req_init).

Aside from my questions the patch looks good.