Re: [RFC PATCH v1 1/7] mm: migrate: Allow misplaced migration without VMA too

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Bharata B Rao <bharata@amd.com> writes:

> We want isolation of misplaced folios to work in contexts
> where VMA isn't available. In order to prepare for that
> allow migrate_misplaced_folio_prepare() to be called with
> a NULL VMA.
>
> Signed-off-by: Bharata B Rao <bharata@amd.com>
> ---
>  mm/migrate.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 425401b2d4e1..7e356c0b1b5a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2619,7 +2619,8 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
>  
>  /*
>   * Prepare for calling migrate_misplaced_folio() by isolating the folio if
> - * permitted. Must be called with the PTL still held.
> + * permitted. Must be called with the PTL still held if called with a non-NULL
> + * vma.

The locking rule is changed.  IMO, it deserves more explanation in patch description.

>   */
>  int migrate_misplaced_folio_prepare(struct folio *folio,
>  		struct vm_area_struct *vma, int node)
> @@ -2636,7 +2637,7 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
>  		 * See folio_maybe_mapped_shared() on possible imprecision
>  		 * when we cannot easily detect if a folio is shared.
>  		 */
> -		if ((vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
> +		if (vma && (vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
>  			return -EACCES;
>  
>  		/*

---
Best Regards,
Huang, Ying