Re: [PATCH] mm/migrate_device: Cleanup up PMD Checks and warnings

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

On 4/14/26 16:13, Sunny Patel wrote:
> Remove the odd VM_WARN_ON_FOLIO(!folio, folio) usage and replace it
> with a simpler VM_WARN_ON_ONCE(!folio) check.
> 
> Drop the redundant VM_WARN_ON_ONCE(!pmd_none(*pmdp) &&
> !is_huge_zero_pmd(*pmdp)).
> 
> Refactor the PMD checks to use pmd_present(), making the control flow
> clearer and avoiding duplicate condition checks.
> 
> Signed-off-by: Sunny Patel <nueralspacetech@gmail.com>
> ---
>  mm/migrate_device.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 8079676c8f1f..fe1c1999aa2d 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -807,8 +807,7 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  	bool flush = false;
>  	unsigned long i;
>  
> -	VM_WARN_ON_FOLIO(!folio, folio);
> -	VM_WARN_ON_ONCE(!pmd_none(*pmdp) && !is_huge_zero_pmd(*pmdp));
> +	VM_WARN_ON_ONCE(!folio);

I'd test for "page" here instead. But it doesn't matter as of today.

>  
>  	if (!thp_vma_suitable_order(vma, addr, HPAGE_PMD_ORDER))
>  		return -EINVAL;
> @@ -865,12 +864,13 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  	if (userfaultfd_missing(vma))
>  		goto unlock_abort;
>  
> -	if (!pmd_none(*pmdp)) {
> +	if (pmd_present(*pmdp)) {
>  		if (!is_huge_zero_pmd(*pmdp))
>  			goto unlock_abort;
>  		flush = true;
> -	} else if (!pmd_none(*pmdp))
> +	} else if (!pmd_none(*pmdp)) {
>  		goto unlock_abort;
> +	}
>  
>  	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>  	folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);

is_huge_zero_pmd() checks pmd_present(), so we didn't have a bug before.

We could also do:

if (is_huge_zero_pmd(*pmdp)) {
	flush = true;
} else if (!pmd_none(*pmdp)) {
	goto unlock_abort;
}


But I don't particularly care

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David