[PATCH v4 2/3] mm/gup: allow FOLL_LONGTERM & FOLL_PCI_P2PDMA

[Martin Oliveira <martin.oliveira@eideticom.com>]

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.

Co-developed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Martin Oliveira <martin.oliveira@eideticom.com>

[1]: https://lkml.kernel.org/r/Yy4Ot5MoOhsgYLTQ@ziepe.ca
---
 mm/gup.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ca0f5cedce9b..6922e1c38d75 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2614,11 +2614,6 @@ static bool is_valid_gup_args(struct page **pages, int *locked,
 	if (WARN_ON_ONCE((gup_flags & (FOLL_GET | FOLL_PIN)) && !pages))
 		return false;
 
-	/* We want to allow the pgmap to be hot-unplugged at all times */
-	if (WARN_ON_ONCE((gup_flags & FOLL_LONGTERM) &&
-			 (gup_flags & FOLL_PCI_P2PDMA)))
-		return false;
-
 	*gup_flags_p = gup_flags;
 	return true;
 }
-- 
2.34.1