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

[Christoph Hellwig]

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 :(