Re: [PATCH v3 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

[Yang Shi <shy828301@gmail.com>]

On Mon, Oct 24, 2022 at 1:34 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
> During THP migration, if THPs are not migrated but they are split and all
> subpages are migrated successfully, migrate_pages() will still return the
> number of THP pages that were not migrated.  This will confuse the callers
> of migrate_pages().  For example, the longterm pinning will failed though
> all pages are migrated successfully.
>
> Thus we should return 0 to indicate that all pages are migrated in this
> case
>
> Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reviewed-by: Alistair Popple <apopple@nvidia.com>
> Cc: <stable@vger.kernel.org>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
> Changes from v2:
>  - Add Fixes tag suggested by Yang Shi and Huang, Ying.
>  - Drop 'nr_thp_split' validation suggested by Alistair.
>  - Add reviewed tag from Alistair.
>  - Update the commit message suggested by Andrew.
> Changes from v1:
>  - Fix the return value of migrate_pages() instead of fixing the
>    callers' validation.
> ---
>  mm/migrate.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8e5eb6e..2eb16f8 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1582,6 +1582,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>          */
>         list_splice(&ret_pages, from);
>
> +       /*
> +        * Return 0 in case all subpages of fail-to-migrate THPs are
> +        * migrated successfully.
> +        */
> +       if (list_empty(from))
> +               rc = 0;
> +
>         count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
>         count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
>         count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
> --
> 1.8.3.1
>