Re: [PATCH v2 14/17] mm/damon: handle device-exclusive entries correctly in damon_folio_young_one()

[SeongJae Park <sj@kernel.org>]

On Mon, 10 Feb 2025 20:37:56 +0100 David Hildenbrand <david@redhat.com> 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 <david@redhat.com>

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


Thanks,
SJ

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