mm/migrate_device: move softleaf_to_folio() inside device-private branch

migrate_vma_collect_pmd() calls softleaf_to_folio() on a non-present PMD
before checking the entry's type.  softleaf_to_folio() converts the
entry's offset to a PFN, which is only meaningful for migration or
device-private entries.

A PMD swap entry's offset is a swap offset, not a PFN, so the lookup would
either return a bogus folio pointer or trip pfn_to_page validation on a
debug kernel.  In the non-device-private path the returned folio is then
unused (the OR short-circuits to migrate_vma_collect_skip()), but the
lookup itself is already unsafe.

Move the softleaf_to_folio() call inside the device-private branch where
the folio is actually needed, mirroring the equivalent
change_non_present_huge_pmd() fix.

Link: https://lore.kernel.org/20260706114320.1643046-6-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 6f8b995810ff..39a6f5230a42 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -166,11 +166,14 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
 	} else if (!pmd_present(*pmdp)) {
 		const softleaf_t entry = softleaf_from_pmd(*pmdp);
 
-		folio = softleaf_to_folio(entry);
-
 		if (!softleaf_is_device_private(entry) ||
-			!(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
-			(folio->pgmap->owner != migrate->pgmap_owner)) {
+		    !(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_PRIVATE)) {
+			spin_unlock(ptl);
+			return migrate_vma_collect_skip(start, end, walk);
+		}
+
+		folio = softleaf_to_folio(entry);
+		if (folio->pgmap->owner != migrate->pgmap_owner) {
 			spin_unlock(ptl);
 			return migrate_vma_collect_skip(start, end, walk);
 		}