Re: [PATCH v2 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

[Alistair Popple <apopple@nvidia.com>]

Baolin Wang <baolin.wang@linux.alibaba.com> writes:

> When THP migration, if THPs are split and all subpages are migrated successfully
> , the migrate_pages() will still return the number of THP that were not migrated.
> That will confuse the callers of migrate_pages(), for example, which will make
> the longterm pinning failed though all pages are migrated successfully.
>
> Thus we should return 0 to indicate all pages are migrated in this case.
>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> 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..1da0dbc 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 (nr_thp_split && list_empty(from))
> +		rc = 0;

Why do you need to check nr_thp_split? Wouldn't list_empty(from) == True
imply success? And if it doesn't imply success wouldn't it be possible
to end up with nr_thp_split && list_empty(from) whilst still having
pages that failed to migrate?

The list management and return code logic from unmap_and_move() has
gotten pretty difficult to follow and could do with some rework IMHO.

>  	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
>  	count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
>  	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);