Re: [PATCH v2 3/3] mm/mempolicy: skip zone device folios when queueing folios

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Aug 17, 2026 at 06:08:10PM -0400, Gregory Price wrote:
> queue_folios_pte_range() already pairs vm_normal_folio() with an
> explicit folio_is_zone_device() check before adding folios to the
> migration pagelist.
>
> vm_normal_folio() alone does not reject zone device memory (a present
> device-coherent page in a normal VMA is returned as "normal").
>
> Mirror the explicit check in queue_folios_pmd() as well.
>
> queue_folios_pmd() uses pmd_folio() directly and can encounter a present
> zone device PMD - e.g. a device-coherent THP.
>
> This is not filtered by existing checks:
>   !pmd_present()   - only rejects non-present device-private and
>                      migration entries
>
>   vma_migratable() - excludes DAX and VM_PFNMAP.
>
> The early return also means such a folio is no longer counted in
> qp->nr_failed under MPOL_MF_STRICT.  This is the same pattern used
> by queue_folios_pte_range() (skipping zone device without failing).
>
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>

Seems sensible so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/mempolicy.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 3498a5651d50f..3418b5664dcf8 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -662,6 +662,8 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
>  		return;
>  	}
>  	folio = pmd_folio(pmdval);
> +	if (folio_is_zone_device(folio))
> +		return;
>  	if (is_huge_zero_folio(folio)) {
>  		walk->action = ACTION_CONTINUE;
>  		return;
> --
> 2.53.0-Meta
>

--
Cheers, Lorenzo