From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bart Van Assche Subject: Re: [PATCH v7 01/13] PCI/P2PDMA: Support peer-to-peer memory Date: Tue, 25 Sep 2018 10:25:40 -0700 Message-ID: <1537896340.11137.19.camel@acm.org> References: <20180925162231.4354-1-logang@deltatee.com> <20180925162231.4354-2-logang@deltatee.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180925162231.4354-2-logang-OTvnGxWRz7hWk0Htik3J/w@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" To: Logan Gunthorpe , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-pci-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nvme-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Cc: Jens Axboe , Christian =?ISO-8859-1?Q?K=F6nig?= , Benjamin Herrenschmidt , Alex Williamson , =?ISO-8859-1?Q?J=E9r=F4me?= Glisse , Jason Gunthorpe , Bjorn Helgaas , Max Gurtovoy , Christoph Hellwig List-Id: linux-rdma@vger.kernel.org On Tue, 2018-09-25 at 10:22 -0600, Logan Gunthorpe wrote: > [ ... ] Hi Logan, It's great to see this patch series making progress. Unfortunately I didn't have the time earlier to have a closer look at this patch series. I hope that you don't mind that I ask a few questions about the implementation? > +static void pci_p2pdma_percpu_kill(void *data) > +{ > + struct percpu_ref *ref = data; > + > + if (percpu_ref_is_dying(ref)) > + return; > + > + percpu_ref_kill(ref); > +} The percpu_ref_is_dying() test should either be removed or a comment should be added above it that explains why it is necessary. Is the purpose of that call perhaps to protect against multiple calls of pci_p2pdma_percpu_kill()? If so, which mechanism serializes these multiple calls? > +static void pci_p2pdma_release(void *data) > +{ > + struct pci_dev *pdev = data; > + > + if (!pdev->p2pdma) > + return; > + > + wait_for_completion(&pdev->p2pdma->devmap_ref_done); > + percpu_ref_exit(&pdev->p2pdma->devmap_ref); > + > + gen_pool_destroy(pdev->p2pdma->pool); > + pdev->p2pdma = NULL; > +} Which code frees the memory pdev->p2pdma points at? Other functions similar to pci_p2pdma_release() call devm_remove_action(), e.g. hmm_devmem_ref_exit(). > +static int pci_p2pdma_setup(struct pci_dev *pdev) > +{ > + int error = -ENOMEM; > + struct pci_p2pdma *p2p; > + > + p2p = devm_kzalloc(&pdev->dev, sizeof(*p2p), GFP_KERNEL); > + if (!p2p) > + return -ENOMEM; > + > + p2p->pool = gen_pool_create(PAGE_SHIFT, dev_to_node(&pdev->dev)); > + if (!p2p->pool) > + goto out; > + > + init_completion(&p2p->devmap_ref_done); > + error = percpu_ref_init(&p2p->devmap_ref, > + pci_p2pdma_percpu_release, 0, GFP_KERNEL); > + if (error) > + goto out_pool_destroy; > + > + percpu_ref_switch_to_atomic_sync(&p2p->devmap_ref); Why are percpu_ref_init() and percpu_ref_switch_to_atomic_sync() called separately instead of passing PERCPU_REF_INIT_ATOMIC to percpu_ref_init()? Would using PERCPU_REF_INIT_ATOMIC eliminate a call_rcu_sched() call and hence make this function faster? > +static struct pci_dev *find_parent_pci_dev(struct device *dev) > +{ > + struct device *parent; > + > + dev = get_device(dev); > + > + while (dev) { > + if (dev_is_pci(dev)) > + return to_pci_dev(dev); > + > + parent = get_device(dev->parent); > + put_device(dev); > + dev = parent; > + } > + > + return NULL; > +} The above function increases the reference count of the device it returns a pointer to. It is a good habit to explain such behavior above the function definition. > +static void seq_buf_print_bus_devfn(struct seq_buf *buf, struct pci_dev *pdev) > +{ > + if (!buf) > + return; > + > + seq_buf_printf(buf, "%s;", pci_name(pdev)); > +} NULL checks in functions that print to a seq buffer are unusual. Is it possible that a NULL pointer gets passed as the first argument to seq_buf_print_bus_devfn()? > +struct pci_p2pdma_client { > + struct list_head list; > + struct pci_dev *client; > + struct pci_dev *provider; > +}; Is there a reason that the peer-to-peer client and server code exist in the same source file? If not, have you considered to split the p2pdma.c file into two files - one with the code for devices that provide p2p functionality and another file with the code that supports p2p users? I think that would make it easier to follow the code. > +/** > + * pci_free_p2pmem - allocate peer-to-peer DMA memory > + * @pdev: the device the memory was allocated from > + * @addr: address of the memory that was allocated > + * @size: number of bytes that was allocated > + */ > +void pci_free_p2pmem(struct pci_dev *pdev, void *addr, size_t size) > +{ > + gen_pool_free(pdev->p2pdma->pool, (uintptr_t)addr, size); > + percpu_ref_put(&pdev->p2pdma->devmap_ref); > +} > +EXPORT_SYMBOL_GPL(pci_free_p2pmem); Please fix the header of this function - there is a copy-paste error in the function header. Thanks, Bart.