[PATCH 6.12.y] mm/rmap: handle device-exclusive entries in folio_referenced_one()

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit f3c493d2f42b5f44a5c134d8d7ea7f960caf5e2c ]

page_vma_mapped_walk() can return non-present device-exclusive entries,
but folio_referenced_one() passes them to PTE young-bit accessors that
require present PTEs.

We must neither call clear_flush_young_ptes_notify() nor
folio_pte_batch() with non-present entries. For example, clearing the
young bit on non-present PTEs can corrupt the entry: on arm64
PTE_AF overlaps the swap type, so clearing it turns a device-exclusive
entry into an ordinary swap entry.

folio_pte_batch() will VM_WARN_ON_FOLIO.

Apply the same approach as the previous fixes to e.g., damon_ptep_mkold():
treat these entries as old from the CPU's perspective and obtain their
young state through the MMU notifier.

Skip PTE batching and MGLRU look-around for non-present entries, and
don't modify non-present PTEs.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-9-d6437171ecc3@kernel.org
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: Adapted batched PTE reference checks to the existing single-PTE walker. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/rmap.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 1bf83e939583f..ad0c6b1eebfec 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -850,6 +850,7 @@ static bool folio_referenced_one(struct folio *folio,
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
 	int referenced = 0;
 	unsigned long start = address, ptes = 0;
+	pte_t pteval;
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		address = pvmw.address;
@@ -892,12 +893,25 @@ static bool folio_referenced_one(struct folio *folio,
 			return false;
 		}
 
-		if (lru_gen_enabled() && pvmw.pte) {
+		if (pvmw.pte)
+			pteval = ptep_get(pvmw.pte);
+
+		if (lru_gen_enabled() && pvmw.pte && pte_present(pteval)) {
 			if (lru_gen_look_around(&pvmw))
 				referenced++;
 		} else if (pvmw.pte) {
-			if (ptep_clear_flush_young_notify(vma, address,
-						pvmw.pte))
+			bool young = false;
+
+			/*
+			 * PFN swap PTEs, such as device-exclusive ones, that
+			 * actually map pages are "old" from a CPU perspective.
+			 * The MMU notifier takes care of any device aspects.
+			 */
+			if (pte_present(pteval))
+				young = ptep_clear_flush_young(vma, address, pvmw.pte);
+			young |= mmu_notifier_clear_flush_young(vma->vm_mm, address,
+								address + PAGE_SIZE);
+			if (young)
 				referenced++;
 		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 			if (pmdp_clear_flush_young_notify(vma, address,
-- 
2.53.0