Re: [PATCH 1/2] mm: gup: Re-pin pages in case of trying several times to migrate

[Huang, Ying]

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

> The migrate_pages() will return the number of {normal page, THP, hugetlb}
> that were not migrated, or an error code. That means it can still return
> the number of failure count, though the pages have been migrated
> successfully with several times re-try.

If my understanding were correct, if pages are migrated successfully
after several times re-tries, the return value will be 0.  There's one
possibility for migrate_pages() to return non-zero but all pages are
migrated.  That is, when THP is split and all subpages are migrated
successfully.

> So we should not use the return value of migrate_pages() to determin
> if there are pages are failed to migrate. Instead we can validate the
> 'movable_page_list' to see if there are pages remained in the list,
> which are failed to migrate. That can mitigate the failure of longterm
> pinning.

Another choice is to use a special return value for split THP + success
migration.  But I'm fine to use list_empty(return_pages).

> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>  mm/gup.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 5182aba..bd8cfcd 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1914,9 +1914,10 @@ static int migrate_longterm_unpinnable_pages(
>  			.gfp_mask = GFP_USER | __GFP_NOWARN,
>  		};
>  
> -		if (migrate_pages(movable_page_list, alloc_migration_target,
> -				  NULL, (unsigned long)&mtc, MIGRATE_SYNC,
> -				  MR_LONGTERM_PIN, NULL)) {
> +		ret = migrate_pages(movable_page_list, alloc_migration_target,
> +				    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
> +				    MR_LONGTERM_PIN, NULL);
> +		if (ret < 0 || !list_empty(movable_page_list)) {

It seems that !list_empty() is sufficient here.

>  			ret = -ENOMEM;

Why change the error code?  I don't think it's a good idea to do that.

>  			goto err;
>  		}

Best Regards,
Huang, Ying