[PATCH v2 13/17] mm/page_idle: handle device-exclusive entries correctly in page_idle_clear_pte_refs_one()

[David Hildenbrand <david@redhat.com>]

Ever since commit b756a3b5e7ea ("mm: device exclusive memory access")
we can return with a device-exclusive entry from page_vma_mapped_walk().

page_idle_clear_pte_refs_one() is not prepared for that, so let's
teach it what to do with these PFN swap PTEs. Note that device-private
entries are so far not applicable on that path, as page_idle_get_folio()
filters out non-lru folios.

Should we just skip PFN swap PTEs completely? Possible, but it seems
straight forward to just handle them correctly.

Note that we could currently only run into this case with
device-exclusive entries on THPs. We still adjust the mapcount on
conversion to device-exclusive; this makes the rmap walk
abort early for small folios, because we'll always have
!folio_mapped() with a single device-exclusive entry. We'll adjust the
mapcount logic once all page_vma_mapped_walk() users can properly
handle device-exclusive entries.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/page_idle.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/page_idle.c b/mm/page_idle.c
index 947c7c7a37289..408aaf29a3ea6 100644
--- a/mm/page_idle.c
+++ b/mm/page_idle.c
@@ -62,9 +62,14 @@ static bool page_idle_clear_pte_refs_one(struct folio *folio,
 			/*
 			 * For PTE-mapped THP, one sub page is referenced,
 			 * the whole THP is referenced.
+			 *
+			 * 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 (ptep_clear_young_notify(vma, addr, pvmw.pte))
-				referenced = true;
+			if (likely(pte_present(ptep_get(pvmw.pte))))
+				referenced |= ptep_test_and_clear_young(vma, addr, pvmw.pte);
+			referenced |= mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE);
 		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 			if (pmdp_clear_young_notify(vma, addr, pvmw.pmd))
 				referenced = true;
-- 
2.48.1