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

[Jason Gunthorpe]

On Sun, Aug 11, 2024 at 11:39:22PM -0700, Christoph Hellwig wrote:
> On Thu, Aug 08, 2024 at 12:33:39PM -0600, Martin Oliveira wrote:
> > This check existed originally due to concerns that P2PDMA needed to copy
> > fsdax until pgmap refcounts were fixed (see [1]).
> > 
> > The P2PDMA infrastructure will only call unmap_mapping_range() when the
> > underlying device is unbound, and immediately after unmapping it waits
> > for the reference of all ZONE_DEVICE pages to be released before
> > continuing. This does not allow for a page to be reused and no user
> > access fault is therefore possible. It does not have the same problem as
> > fsdax.
> > 
> > The one minor concern with FOLL_LONGTERM pins is they will block device
> > unbind until userspace releases them all.
> 
> 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?

Jason