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

[Logan Gunthorpe]

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.

Logan