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

[Jason Gunthorpe]

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