Re: [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-mm@kvack.org, iommu@lists.linux-foundation.org,
	"Stephen Bates" <sbates@raithlin.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Christian König" <christian.koenig@amd.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Don Dutile" <ddutile@redhat.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Jakowski Andrzej" <andrzej.jakowski@intel.com>,
	"Minturn Dave B" <dave.b.minturn@intel.com>,
	"Jason Ekstrand" <jason@jlekstrand.net>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Xiong Jianxin" <jianxin.xiong@intel.com>,
	"Bjorn Helgaas" <helgaas@kernel.org>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Martin Oliveira" <martin.oliveira@eideticom.com>,
	"Chaitanya Kulkarni" <ckulkarnilinux@gmail.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>
Subject: Re: [PATCH v6 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()
Date: Thu, 2 Jun 2022 14:28:11 -0300	[thread overview]
Message-ID: <20220602172811.GN2960187@ziepe.ca> (raw)
In-Reply-To: <51cc6645-2504-6d01-5626-238ed6ffff04@deltatee.com>

On Thu, Jun 02, 2022 at 10:49:15AM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2022-06-02 10:30, Jason Gunthorpe wrote:
> > On Thu, Jun 02, 2022 at 10:16:10AM -0600, Logan Gunthorpe wrote:
> > 
> >>> Just stuff the pages into the mmap, and your driver unprobe will
> >>> automatically block until all the mmaps are closed - no different than
> >>> having an open file descriptor or something.
> >>
> >> Oh is that what we want?
> > 
> > Yes, it is the typical case - eg if you have a sysfs file open unbind
> > hangs indefinitely. Many drivers can't unbind while they have open file
> > descriptors/etc.
> > 
> > A couple drivers go out of their way to allow unbinding while a live
> > userspace exists but this can get complicated. Usually there should be
> > a good reason.
> > 
> > The module will already be refcounted anyhow because the mmap points
> > to a char file which holds a module reference - meaning a simple rmmod
> > of the driver shouldn't work already..
> 
> Also, I just tried it... If I open a sysfs file for an nvme device (ie.
> /sys/class/nvme/nvme4/cntlid) and unbind the device, it does not block.
> A subsequent read on that file descriptor returns ENODEV. Which is what
> I would have expected.

Oh interesting, this has been changed since years ago when I last
looked, the kernfs_get_active() is now more narrowed than it once
was. So manybe sysfs isn't the same concern it used to be!

Thanks,
Jason