mm/damon: handle device-exclusive entries correctly in damon_folio_young_one()

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

damon_folio_young_one() is not prepared for that, so teach it about these
PFN swap PTEs.  Note that device-private entries are so far not applicable
on that path, as we expect ZONE_DEVICE pages so far only in migration code
when it comes to the RMAP.

The impact is rather small: we'd be calling pte_young() on a non-present
PTE, which is not really defined to have semantic.

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.

Link: https://lkml.kernel.org/r/20250210193801.781278-15-david@redhat.com
Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Tested-by: Alistair Popple <apopple@nvidia.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Lyude <lyude@redhat.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yanteng Si <si.yanteng@linux.dev>
Cc: Barry Song <v-songbaohua@oppo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index c834aa217835..1a89920efce9 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -92,12 +92,20 @@ static bool damon_folio_young_one(struct folio *folio,
 {
 	bool *accessed = arg;
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, addr, 0);
+	pte_t pte;
 
 	*accessed = false;
 	while (page_vma_mapped_walk(&pvmw)) {
 		addr = pvmw.address;
 		if (pvmw.pte) {
-			*accessed = pte_young(ptep_get(pvmw.pte)) ||
+			pte = ptep_get(pvmw.pte);
+
+			/*
+			 * 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.
+			 */
+			*accessed = (pte_present(pte) && pte_young(pte)) ||
 				!folio_test_idle(folio) ||
 				mmu_notifier_test_young(vma->vm_mm, addr);
 		} else {