Re: [PATCH 1/4] migrate: Allow migrate_misplaced_folio APIs without a VMA

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

Hi, Gregory,

Gregory Price <gourry@gourry.net> writes:

> To migrate unmapped pagecache folios, migrate_misplaced_folio and
> migrate_misplaced_folio_prepare must handle folios without VMAs.

IMHO, it's better to use migrate_misplaced_folio() instead of
migrate_misplaced_folio for readability in patch title and description.

> migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
> a NULL VMA when it does not have a mapping.
>
> migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
> when in the pagecache path because it is a synchronous context.

I don't find the corresponding implementation for this.  And, I don't
think it's a good idea to change from MIGRATE_ASYNC to MIGRATE_SYNC.
This may cause too long page access latency for page placement
optimization.  The downside may offset the benefit.

And, it appears that we can delete the "vma" parameter of
migrate_misplaced_folio() because it's not used now.  This is a trivial
code cleanup.

> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dfb5eba3c522..3b0bd3f21ac3 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2632,7 +2632,7 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
>  		 * See folio_likely_mapped_shared() on possible imprecision
>  		 * when we cannot easily detect if a folio is shared.
>  		 */
> -		if ((vma->vm_flags & VM_EXEC) &&
> +		if (vma && (vma->vm_flags & VM_EXEC) &&
>  		    folio_likely_mapped_shared(folio))
>  			return -EACCES;

---
Best Regards,
Huang, Ying