[PATCH 09/46] mm/rmap: handle device-exclusive entries in folio_referenced_one()

[David Hildenbrand (Arm)]

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 at kernel.org>
Cc: stable at vger.kernel.org
Reported-by: Josef Bacik <josef at toxicpanda.com>
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/rmap.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 1131b76bbbc28..0faef46806e5c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -924,6 +924,7 @@ static bool folio_referenced_one(struct folio *folio,
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
 	int ptes = 0, referenced = 0;
 	unsigned int nr;
+	pte_t pteval;
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		address = pvmw.address;
@@ -968,10 +969,12 @@ static bool folio_referenced_one(struct folio *folio,
 			return false;
 		}
 
-		if (pvmw.pte && folio_test_large(folio)) {
+		if (pvmw.pte)
+			pteval = ptep_get(pvmw.pte);
+
+		if (pvmw.pte && pte_present(pteval) && folio_test_large(folio)) {
 			const unsigned long end_addr = pmd_addr_end(address, vma->vm_end);
 			const unsigned int max_nr = (end_addr - address) >> PAGE_SHIFT;
-			pte_t pteval = ptep_get(pvmw.pte);
 
 			nr = folio_pte_batch(folio, pvmw.pte, pteval, max_nr);
 		}
@@ -981,11 +984,23 @@ static bool folio_referenced_one(struct folio *folio,
 		 * are. —they could be on active/inactive lists or on MGLRU. So the
 		 * simplest approach is to disable this look-around optimization.
 		 */
-		if (lru_gen_enabled() && !lru_gen_switching() && pvmw.pte) {
+		if (lru_gen_enabled() && !lru_gen_switching() && pvmw.pte &&
+		    pte_present(pteval)) {
 			if (lru_gen_look_around(&pvmw, nr))
 				referenced++;
 		} else if (pvmw.pte) {
-			if (clear_flush_young_ptes_notify(vma, address, pvmw.pte, nr))
+			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 = clear_flush_young_ptes(vma, address, pvmw.pte, nr);
+			young |= mmu_notifier_clear_flush_young(vma->vm_mm, address,
+								address + nr * PAGE_SIZE);
+			if (young)
 				referenced++;
 		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 			if (pmdp_clear_flush_young_notify(vma, address,

-- 
2.43.0