On Mon, 10 Feb 2025 20:37:56 +0100 David Hildenbrand [off-list ref] wrote:

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.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <redacted>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

[...]