Re: [PATCH 6/7] migrate_pages(): fix failure counting for THP splitting

[Baolin Wang]

On 6/27/2022 10:24 AM, Huang Ying wrote:
> If THP is failed to be migrated, it may be split and retry.  But after
> splitting, the head page will be left in "from" list, although THP
> migration failure has been counted already.  If the head page is
> failed to be migrated too, the failure will be counted twice
> incorrectly.  So this is fixed in this patch via moving the head page
> of THP after splitting to "thp_split_pages" too.
>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Fixes: 5984fabb6e82 ("mm: move_pages: report the number of non-attempted pages")
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> ---

Good catch. LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

>   mm/migrate.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 9d1883d5927f..70a0e1f34c03 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1305,6 +1305,8 @@ static inline int try_split_thp(struct page *page, struct list_head *split_pages
>   	lock_page(page);
>   	rc = split_huge_page_to_list(page, split_pages);
>   	unlock_page(page);
> +	if (!rc)
> +		list_move_tail(&page->lru, split_pages);
>   
>   	return rc;
>   }
> @@ -1364,7 +1366,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   		thp_retry = 0;
>   
>   		list_for_each_entry_safe(page, page2, from, lru) {
> -retry:
>   			/*
>   			 * THP statistics is based on the source huge page.
>   			 * Capture required information that might get lost
> @@ -1411,7 +1412,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   					nr_thp_failed++;
>   					if (!try_split_thp(page, &thp_split_pages)) {
>   						nr_thp_split++;
> -						goto retry;
> +						break;
>   					}
>   				/* Hugetlb migration is unsupported */
>   				} else if (!no_subpage_counting) {
> @@ -1431,7 +1432,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   					/* THP NUMA faulting doesn't split THP to retry. */
>   					if (!nosplit && !try_split_thp(page, &thp_split_pages)) {
>   						nr_thp_split++;
> -						goto retry;
> +						break;
>   					}
>   				} else if (!no_subpage_counting) {
>   					nr_failed++;