[v2,12/17] mm/rmap: handle device-exclusive entries correctly in page_vma_mkclean_one()

Commit Message

David Hildenbrand Feb. 10, 2025, 7:37 p.m. UTC

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

page_vma_mkclean_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.

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 <[e-mail]>
---
 mm/rmap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Patch

diff --git a/mm/rmap.c b/mm/rmap.c
index 7c471c3ea64c4..7b737f0f68fb5 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1044,6 +1044,14 @@  static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
 			pte_t *pte = pvmw->pte;
 			pte_t entry = ptep_get(pte);
 
+			/*
+			 * PFN swap PTEs, such as device-exclusive ones, that
+			 * actually map pages are clean and not writable from a
+			 * CPU perspective. The MMU notifier takes care of any
+			 * device aspects.
+			 */
+			if (!pte_present(entry))
+				continue;
 			if (!pte_dirty(entry) && !pte_write(entry))
 				continue;