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

[Jason Gunthorpe]

On Thu, Jun 02, 2022 at 10:45:55AM -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.
> 
> This is not my experience. All the drivers I've worked with do not block
> unbind with open file descriptors (at least for char devices). I know,
> for example, that having a file descriptor open of /dev/nvmeX does not
> cause unbinding to block.

So there are lots of bugs in the kernel, and I've seen many drivers
that think calling cdev_device_del() is all they need to do - and then
happily allow cdev ioctl's/etc on a de-initialized driver struct.

Drivers that do take care of this usually have to put a lock around
all their fops to serialize against unbind. RDMA uses SRCU, iirc TPM
used a rwlock. But this is tricky and hurts fops performance.

I don't know what nvme did to protect against this, I didn't notice
an obvious lock.

> I figured this was the expectation as the userspace process doing
> the unbind won't be able to be interrupted seeing there's no way to
> fail on that path. Though, it certainly would make things a lot
> easier if the unbind can block indefinitely as it usually requires
> some complicated locking.

As I said, this is what sysfs does today and I don't see that ever
changing. If you userspace has a sysfs file open then the driver
unbind hangs until the file is closed.

So, doing as bad as sysfs seems like a reasonable baseline to me.

> Do you have an example of this? What mechanisms are developers using to
> block unbind with open file descriptors?

Sysfs maintains a refcount with a bias that is basically a fancied
rwlock. Most places use some kind of refcount triggering a
completion. Sleep on the completion until refcount is 0 on unbind kind
of thing.

Jason