Re: [PATCH v5 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI_P2PDMA

[Dan Williams <dan.j.williams@intel.com>]

Jason Gunthorpe wrote:
> On Mon, Aug 12, 2024 at 10:41:20PM -0700, Christoph Hellwig wrote:
> > On Mon, Aug 12, 2024 at 08:12:49PM -0300, Jason Gunthorpe wrote:
> > > > This is unfortunately not really minor unless we have a well documented
> > > > way to force this :(
> > > 
> > > It is not that different from blocking driver unbind while FDs are
> > > open which a lot of places do in various ways?
> > 
> > Where do we block driver unbind with an open resource?  
> 
> I keep seeing it in different subsystems, safe driver unbind is really
> hard. :\ eg I think VFIO has some waits in it
> 
> > The whole concept is that open resources will pin the in-memory
> > object (and modulo for a modular driver), but never an unbind or
> > hardware unplug, of which unbind really just is a simulation.
> 
> Yes, ideally, but not every part of the kernel hits that ideal in my
> experience. It is alot of work and some places don't have any good
> solutions, like here.

...but there is a distinction between transient and permanent waits,
right? The difficult aspect of FOLL_LONGTERM is the holder has no idea
someone is trying to cleanup and may never drop its pin.