[PATCH 4/6] mm/gup: handle ZONE_DEVICE pages in folio_fast_pin_allowed()

[Martin Oliveira]

folio_fast_pin_allowed() does not support ZONE_DEVICE pages because
currently it is impossible for that type of page to be used with
FOLL_LONGTERM. When this changes in a subsequent patch, this path will
attempt to read the mapping of a ZONE_DEVICE page which is not valid.

Instead, allow ZONE_DEVICE pages explicitly seeing they shouldn't pose
any problem with the fast path.

Co-developed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Signed-off-by: Martin Oliveira <martin.oliveira@xxxxxxxxxxxxx>
---
 mm/gup.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index ca0f5cedce9b..00d0a77112f4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2847,6 +2847,10 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 	if (folio_test_hugetlb(folio))
 		return true;
 
+	/* It makes no sense to access the mapping of ZONE_DEVICE pages */
+	if (folio_is_zone_device(folio))
+		return true;
+
 	/*
 	 * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
 	 * cannot proceed, which means no actions performed under RCU can
-- 
2.34.1