[PATCH v3 1/3] mm/mempolicy: skip non-present PMDs when queueing folios

[Usama Arif <usama.arif@linux.dev>]

queue_folios_pmd() is called under pmd_trans_huge_lock(), whose
pmd_is_huge() check returns true for any non-present, non-none PMD
softleaf. Passing such a PMD to pmd_folio() treats the softleaf encoding
as a hardware PFN and can return a bogus folio pointer.

Mirror queue_folios_pte_range(): handle non-present entries before
looking up a folio. Keep migration entries counted as failures, but skip
other non-present PMDs such as device-private entries.

Potential trigger: an HMM-based GPU driver migrates an anonymous THP
folio to device memory via migrate_vma_pages(), leaving a device-private
PMD. Userspace then calls mbind(), migrate_pages() or
set_mempolicy_home_node() on that range.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=6
Fixes: 368076f52ebe ("mm/huge_memory: add device-private THP support to PMD operations")
Cc: <stable@vger.kernel.org>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/mempolicy.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 914f81863db5..69a00a324ef5 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -654,12 +654,14 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
 {
 	struct folio *folio;
 	struct queue_pages *qp = walk->private;
+	pmd_t pmdval = pmdp_get(pmd);
 
-	if (unlikely(pmd_is_migration_entry(*pmd))) {
-		qp->nr_failed++;
+	if (unlikely(!pmd_present(pmdval))) {
+		if (pmd_is_migration_entry(pmdval))
+			qp->nr_failed++;
 		return;
 	}
-	folio = pmd_folio(*pmd);
+	folio = pmd_folio(pmdval);
 	if (is_huge_zero_folio(folio)) {
 		walk->action = ACTION_CONTINUE;
 		return;
-- 
2.53.0-Meta