Re: [PATCH 2/8] migrate_pages: separate hugetlb folios migration

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

On Tue, 27 Dec 2022 08:28:53 +0800 Huang Ying <ying.huang@intel.com> wrote:

> This is a preparation patch to batch the folio unmapping and moving
> for the non-hugetlb folios.  Based on that we can batch the TLB
> shootdown during the folio migration and make it possible to use some
> hardware accelerator for the folio copying.
> 
> In this patch the hugetlb folios and non-hugetlb folios migration is
> separated in migrate_pages() to make it easy to change the non-hugetlb
> folios migration implementation.
> 
> ...
>
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1404,6 +1404,87 @@ struct migrate_pages_stats {
>  	int nr_thp_split;
>  };
>  
> +static int migrate_hugetlbs(struct list_head *from, new_page_t get_new_page,
> +			    free_page_t put_new_page, unsigned long private,
> +			    enum migrate_mode mode, int reason,
> +			    struct migrate_pages_stats *stats,
> +			    struct list_head *ret_folios)
> +{
> +	int retry = 1;
> +	int nr_failed = 0;
> +	int nr_retry_pages = 0;
> +	int pass = 0;
> +	struct folio *folio, *folio2;
> +	int rc = 0, nr_pages;
> +
> +	for (pass = 0; pass < 10 && retry; pass++) {

Why 10?

> +		retry = 0;
> +		nr_retry_pages = 0;
> +
> +		list_for_each_entry_safe(folio, folio2, from, lru) {
> +			if (!folio_test_hugetlb(folio))
> +				continue;
> +
> +			nr_pages = folio_nr_pages(folio);
> +
> +			cond_resched();
> +
> +			rc = unmap_and_move_huge_page(get_new_page,
> +						      put_new_page, private,
> +						      &folio->page, pass > 2, mode,
> +						      reason, ret_folios);
> +			/*
> +			 * The rules are:
> +			 *	Success: hugetlb folio 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_folios list
> +			 */
> +			switch(rc) {
> +			case -ENOSYS:
> +				/* Hugetlb migration is unsupported */
> +				nr_failed++;
> +				stats->nr_failed_pages += nr_pages;
> +				list_move_tail(&folio->lru, ret_folios);
> +				break;
> +			case -ENOMEM:
> +				/*
> +				 * When memory is low, don't bother to try to migrate
> +				 * other folios, just exit.
> +				 */
> +				nr_failed++;
> +				stats->nr_failed_pages += nr_pages;
> +				goto out;
> +			case -EAGAIN:
> +				retry++;
> +				nr_retry_pages += nr_pages;
> +				break;
> +			case MIGRATEPAGE_SUCCESS:
> +				stats->nr_succeeded += nr_pages;
> +				break;
> +			default:
> +				/*
> +				 * Permanent failure (-EBUSY, etc.):
> +				 * unlike -EAGAIN case, the failed folio is
> +				 * removed from migration folio list and not
> +				 * retried in the next outer loop.
> +				 */
> +				nr_failed++;
> +				stats->nr_failed_pages += nr_pages;
> +				break;
> +			}
> +		}
> +	}
> +out:
> +	nr_failed += retry;
> +	stats->nr_failed_pages += nr_retry_pages;
> +	if (rc != -ENOMEM)
> +		rc = nr_failed;
> +
> +	return rc;
> +}

The interpretation of the return value of this function is somewhat
unobvious.

I suggest that this function be fully commented.

Why does a retry contribute to nr_failed.  What is the interpretation
of nr_failed.  etcetera.