Re: [PATCH -V2 5/7] migrate_pages(): fix failure counting for THP on -ENOSYS

[Oscar Salvador]

On Mon, Jul 11, 2022 at 04:49:46PM +0800, Huang Ying wrote:
> If THP or hugetlbfs page migration isn't supported, unmap_and_move()
> or unmap_and_move_huge_page() will return -ENOSYS.  For THP, splitting
> will be tried, but if splitting doesn't succeed, the THP will be left
> in "from" list wrongly.  If some other pages are retried, the THP
> migration failure will counted again.  This is fixed via moving the
> failure THP from "from" to "ret_pages".
> 
> Another issue of the original code is that the unsupported failure
> processing isn't consistent between THP and hugetlbfs page.  Make them
> consistent in this patch to make the code easier to be understood too.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Fixes: 5984fabb6e82 ("mm: move_pages: report the number of non-attempted pages")
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> ---
>  mm/migrate.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 4bceba143db0..8cce73b7c046 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1192,10 +1192,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  	 * tables or check whether the hugepage is pmd-based or not before
>  	 * kicking migration.
>  	 */
> -	if (!hugepage_migration_supported(page_hstate(hpage))) {
> -		list_move_tail(&hpage->lru, ret);
> +	if (!hugepage_migration_supported(page_hstate(hpage)))
>  		return -ENOSYS;
> -	}
>  
>  	if (page_count(hpage) == 1) {
>  		/* page was freed from under us. So we are done. */
> @@ -1392,6 +1390,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  			 *		 page will be put back
>  			 *	-EAGAIN: stay on the from list
>  			 *	-ENOMEM: stay on the from list
> +			 *	-ENOSYS: stay on the from list
>  			 *	Other errno: put on ret_pages list then splice to
>  			 *		     from list
>  			 */
> @@ -1421,6 +1420,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				}
>  
>  				nr_failed_pages += nr_subpages;
> +				list_move_tail(&page->lru, &ret_pages);

I must be missing something, but migrate_pages() calls unmap_and_move_huge_page() 
with ret being ret_pages, so

list_move_tail(&hpage->lru, ret) == list_move_tail(&page->lru, &ret_pages)

Yet, you say "This is fixed via moving the failure THP from "from" to "ret_pages"".
/me confused.


-- 
Oscar Salvador
SUSE Labs