mm/rmap: handle device-exclusive entries correctly in page_vma_mkclean_one()

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.

Link: https://lkml.kernel.org/r/20250210193801.781278-13-david@redhat.com
Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@redhat.com>
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: SeongJae Park <sj@kernel.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, 8 insertions, 0 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 7c471c3ea64c..7b737f0f68fb 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;