Re: [PATCH 2/3] mm: migrate: Correct the hugetlb migration stats

[Andrew Morton <akpm@linux-foundation.org>]

On Sun,  7 Nov 2021 16:57:26 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:

> Correct the migration stats for hugetlb with using compound_nr() instead
> of thp_nr_pages(),

It would be helpful to explain why using thp_nr_pages() was wrong.
And to explain the end user visible effects of this bug so we can
decide whether -stable backporting is desirable.

> meanwhile change 'nr_failed_pages' to record the
> number of normal pages failed to migrate, including THP and hugetlb,
> and 'nr_succeeded' will record the number of normal pages migrated
> successfully.
> 
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/migrate.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 9aafdab..756190b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1436,9 +1436,9 @@ static inline int try_split_thp(struct page *page, struct page **page2,
>   * It is caller's responsibility to call putback_movable_pages() to return pages
>   * to the LRU or free list only if ret != 0.
>   *
> - * Returns the number of {normal page, THP} that were not migrated, or an error code.
> - * The number of THP splits will be considered as the number of non-migrated THP,
> - * no matter how many subpages of the THP are migrated successfully.
> + * Returns the number of {normal page, THP, hugetlb} that were not migrated, or

This is a bit confusing.

If migrate_pages() failed to migrate one 4k pages then it returns 1,
yes?

But if migrate_pages() fails to migrate one 2MB hugepage then will it
return 1 or will it return 512?

And how can the caller actually _use_ this return value without knowing
the above information?

In other words, perhaps this function should simply return pass/fail?