Re: [PATCH 12/16] mm/migration: fix potential page refcounts leak in migrate_pages

[Baolin Wang]

Hi Miaohe,

On 3/4/2022 5:34 PM, Miaohe Lin wrote:
> In -ENOMEM case, there might be some subpages of fail-to-migrate THPs
> left in thp_split_pages list. We should move them back to migration
> list so that they could be put back to the right list by the caller
> otherwise the page refcnt will be leaked here. Also adjust nr_failed
> and nr_thp_failed accordingly to make vm events account more accurate.
>
> Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>   mm/migrate.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e0db06927f02..6c2dfed2ddb8 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1422,6 +1422,15 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   				}
>   
>   				nr_failed_pages += nr_subpages;
> +				/*
> +				 * There might be some subpages of fail-to-migrate THPs
> +				 * left in thp_split_pages list. Move them back to migration
> +				 * list so that they could be put back to the right list by
> +				 * the caller otherwise the page refcnt will be leaked.
> +				 */
> +				list_splice_init(&thp_split_pages, from);
> +				nr_failed += retry;
> +				nr_thp_failed += thp_retry;

Yes, I think we missed this case before, and your patch looks right. But 
we should also update the 'rc' to return the correct number of pages 
that were not migrated, right?