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

[Zi Yan <ziy@nvidia.com>]

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On 29 Oct 2021, at 3:42, Baolin Wang wrote:

> Now hugetlb migration is also available for some scenarios, such as
> soft offling or memory compaction. So we should correct the migration

hugetlb migration is available at the time if (PageHuge(page)) branch
is added. I am not sure what is new here.

> stats for hugetlb with using compound_nr() instead of thp_nr_pages()
> to get the number of pages.

nr_failed records the number of pages, not subpages. It is returned to
user space when move_pages() syscall is used. After your change,
if users try to migrate a list of pages including THPs and/or hugetlb
pages and some of THPs and/or hugetlb fail to migrate, move_pages()
will return a number larger than the number of pages the users tried
to migrate. I am not sure this is the change we want. Or at least,
the comment of migrate_pages() and the manpage of move_pages() need
to be changed and linux-api mailing list should be cc’d.

>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/migrate.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a11e948..2b45a29 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1475,7 +1475,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  			 * during migration.
>  			 */
>  			is_thp = PageTransHuge(page) && !PageHuge(page);
> -			nr_subpages = thp_nr_pages(page);
> +			nr_subpages = compound_nr(page);
>  			cond_resched();
>
>  			if (PageHuge(page))
> @@ -1540,7 +1540,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  					nr_failed += nr_subpages;
>  					goto out;
>  				}
> -				nr_failed++;
> +				nr_failed += nr_subpages;
>  				goto out;
>  			case -EAGAIN:
>  				if (is_thp) {
> @@ -1550,14 +1550,14 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				retry++;
>  				break;
>  			case MIGRATEPAGE_SUCCESS:
> +				nr_succeeded += nr_subpages;
>  				if (is_thp) {
>  					nr_thp_succeeded++;
> -					nr_succeeded += nr_subpages;
>  					break;
>  				}
> -				nr_succeeded++;
>  				break;
>  			default:
> +				nr_failed += nr_subpages;
>  				/*
>  				 * Permanent failure (-EBUSY, etc.):
>  				 * unlike -EAGAIN case, the failed page is
> @@ -1566,10 +1566,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				 */
>  				if (is_thp) {
>  					nr_thp_failed++;
> -					nr_failed += nr_subpages;
>  					break;
>  				}
> -				nr_failed++;
>  				break;
>  			}
>  		}
> -- 
> 1.8.3.1

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Best Regards,
Yan, Zi

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