Re: [PATCH 4/4] mm/mempolicy: skip zone device folios when queueing folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/28/26 21:47, 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 the two other walkers.
> 
> 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).
> 
> queue_folios_hugetlb() cannot see zone device memory - hugetlb folios
> are never ZONE_DEVICE - so the check there is purely defensive and keeps
> all three walkers consistent.
> 

No fixes tag?

> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
> ---
>  mm/mempolicy.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 5720f7f54d94..bd79e61a40d1 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -668,6 +668,8 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
>  	}
>  	if (!queue_folio_required(folio, qp))
>  		return;
> +	if (folio_is_zone_device(folio))
> +		return;

Why not before the queue_folio_required() check like the PMD case does?

>  	if (!(qp->flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) ||
>  	    !vma_migratable(walk->vma) ||
>  	    !migrate_folio_add(folio, qp->pagelist, qp->flags))
> @@ -797,6 +799,8 @@ static int queue_folios_hugetlb(pte_t *pte, unsigned long hmask,
>  	folio = pfn_folio(pte_pfn(ptep));
>  	if (!queue_folio_required(folio, qp))
>  		goto unlock;
> +	if (folio_is_zone_device(folio))
> +		goto unlock;

Leave the hugetlb part alone. hugetlb folios will never be ZONE_DEVICE.

-- 
Cheers,

David