[PATCH v3 4/6] mm/migrate: use huge_ptep_get() in remove_migration_pte()

[Dev Jain]

remove_migration_pte() converts migration entries back to present PTEs
after folio migration completes. For hugetlb folios,
page_vma_mapped_walk() returns the pte pointer to the hugetlb folio in
pvmw.pte, but the code reads it with ptep_get().

On arches which provide their own huge_ptep_get() to dereference a huge
pte pointer, accessing via ptep_get() would cause pte_pfn(),
pte_present() etc to misbehave.

It is not clear whether this has a trivially visible effect to userspace.

Use huge_ptep_get() to dereference a huge pte pointer.

Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
Cc: stable at vger.kernel.org
Acked-by: Muchun Song <muchun.song at linux.dev>
Acked-by: David Hildenbrand (Arm) <david at kernel.org>
Signed-off-by: Dev Jain <dev.jain at arm.com>
---
 mm/migrate.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index d9b23909d716c..c65f0f43df7eb 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -371,7 +371,11 @@ static bool remove_migration_pte(struct folio *folio,
 			continue;
 		}
 #endif
-		old_pte = ptep_get(pvmw.pte);
+		if (folio_test_hugetlb(folio))
+			old_pte = huge_ptep_get(vma->vm_mm, pvmw.address,
+						pvmw.pte);
+		else
+			old_pte = ptep_get(pvmw.pte);
 		if (rmap_walk_arg->map_unused_to_zeropage &&
 		    try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
 			continue;
-- 
2.43.0