Re: [PATCH v3 09/47] mm/rmap: handle device-exclusive entries in folio_referenced_one()

[Baolin Wang]

On 10/31/26 5:39 PM, David Hildenbrand (Arm) wrote:
> page_vma_mapped_walk() can return non-present device-exclusive entries,
> but folio_referenced_one() passes them to PTE young-bit accessors that
> require present PTEs.
>
> We must neither call clear_flush_young_ptes_notify() nor
> folio_pte_batch() with non-present entries. For example, clearing the
> young bit on non-present PTEs can corrupt the entry: on arm64
> PTE_AF overlaps the swap type, so clearing it turns a device-exclusive
> entry into an ordinary swap entry.
>
> folio_pte_batch() will VM_WARN_ON_FOLIO.
>
> Apply the same approach as the previous fixes to e.g., damon_ptep_mkold():
> treat these entries as old from the CPU's perspective and obtain their
> young state through the MMU notifier.
>
> Skip PTE batching and MGLRU look-around for non-present entries, and
> don't modify non-present PTEs.
>
> Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Reviewed-by: Harry Yoo (Meta) <harry@xxxxxxxxxx>
> Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
> Reviewed-by: Dev Jain <dev.jain@xxxxxxx>
> Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>