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

Andrew Morton
2022-12-28T23:17:43Z

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.