linux-kernel - Re: [RFC PATCH 15/15] nvme-pci: Allow mmaping the CMB in userspace

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Date:   Fri, 6 Nov 2020 13:39:32 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        Ira Weiny <iweiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC PATCH 15/15] nvme-pci: Allow mmaping the CMB in userspace

On Fri, Nov 06, 2020 at 10:00:36AM -0700, Logan Gunthorpe wrote:
> Allow userspace to obtain CMB memory by mmaping the controller's
> char device. The mmap call allocates and returns a hunk of CMB memory,
> (the offset is ignored) so userspace does not have control over the
> address within the CMB.
> 
> A VMA allocated in this way will only be usable by drivers that set
> FOLL_PCI_P2PDMA when calling GUP. And inter-device support will be
> checked the first time the pages are mapped for DMA.
> 
> Currently this is only supported by O_DIRECT to an PCI NVMe device
> or through the NVMe passthrough IOCTL.
> 
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>  drivers/nvme/host/core.c | 11 +++++++++++
>  drivers/nvme/host/nvme.h |  1 +
>  drivers/nvme/host/pci.c  |  9 +++++++++
>  3 files changed, 21 insertions(+)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index f14316c9b34a..fc642aba671d 100644
> +++ b/drivers/nvme/host/core.c
> @@ -3240,12 +3240,23 @@ static long nvme_dev_ioctl(struct file *file, unsigned int cmd,
>  	}
>  }
>  
> +static int nvme_dev_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	struct nvme_ctrl *ctrl = file->private_data;
> +
> +	if (!ctrl->ops->mmap_cmb)
> +		return -ENODEV;
> +
> +	return ctrl->ops->mmap_cmb(ctrl, vma);
> +}

This needs to ensure that the VMA created is destroyed before the
driver is unprobed - ie the struct pages backing the BAR memory is
destroyed.

I don't see anything that synchronizes this in the nvme_dev_release()?

Many places do this by putting all the VMAs into an address space and
zaping the address space when unprobing the driver to revoke the
pages, but there is a tricky race here :\

https://lore.kernel.org/dri-devel/20201021125030.GK36674@ziepe.ca/

Jason