Re: [RFC 1/6] mm/migrate_pages: separate huge page and normal pages migration

[Zi Yan <ziy@nvidia.com>]

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On 21 Sep 2022, at 2:06, Huang Ying wrote:

> This is a preparation patch to batch the page unmapping and moving for
> the normal pages and THPs.  Based on that we can batch the TLB
> shootdown during the page migration and make it possible to use some
> hardware accelerator for the page copying.
>
> In this patch the huge page (PageHuge()) and normal page and THP
> migration is separated in migrate_pages() to make it easy to change
> the normal page and THP migration implementation.
>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Matthew Wilcox <willy@infradead.org>
> ---
>  mm/migrate.c | 73 +++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 64 insertions(+), 9 deletions(-)

Maybe it would be better to have two subroutines for hugetlb migration
and normal page migration respectively. migrate_pages() becomes very
large at this point.

>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 571d8c9fd5bc..117134f1c6dc 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1414,6 +1414,66 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>
>  	trace_mm_migrate_pages_start(mode, reason);
>
> +	for (pass = 0; pass < 10 && retry; pass++) {
> +		retry = 0;
> +
> +		list_for_each_entry_safe(page, page2, from, lru) {
> +			nr_subpages = compound_nr(page);
> +			cond_resched();
> +
> +			if (!PageHuge(page))
> +				continue;
> +
> +			rc = unmap_and_move_huge_page(get_new_page,
> +						put_new_page, private, page,
> +						pass > 2, mode, reason,
> +						&ret_pages);
> +			/*
> +			 * The rules are:
> +			 *	Success: hugetlb 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
> +			 */
> +			switch(rc) {
> +			case -ENOSYS:
> +				/* Hugetlb migration is unsupported */
> +				nr_failed++;
> +				nr_failed_pages += nr_subpages;
> +				list_move_tail(&page->lru, &ret_pages);
> +				break;
> +			case -ENOMEM:
> +				/*
> +				 * When memory is low, don't bother to try to migrate
> +				 * other pages, just exit.
> +				 */
> +				nr_failed++;
> +				nr_failed_pages += nr_subpages + nr_retry_pages;
> +				goto out;
> +			case -EAGAIN:
> +				retry++;
> +				nr_retry_pages += nr_subpages;
> +				break;
> +			case MIGRATEPAGE_SUCCESS:
> +				nr_succeeded += nr_subpages;
> +				break;
> +			default:
> +				/*
> +				 * Permanent failure (-EBUSY, etc.):
> +				 * unlike -EAGAIN case, the failed page is
> +				 * removed from migration page list and not
> +				 * retried in the next outer loop.
> +				 */
> +				nr_failed++;
> +				nr_failed_pages += nr_subpages;
> +				break;
> +			}
> +		}
> +	}
> +	nr_failed += retry;
> +	retry = 1;
>  thp_subpage_migration:
>  	for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
>  		retry = 0;
> @@ -1431,18 +1491,14 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  			cond_resched();
>
>  			if (PageHuge(page))
> -				rc = unmap_and_move_huge_page(get_new_page,
> -						put_new_page, private, page,
> -						pass > 2, mode, reason,
> -						&ret_pages);
> -			else
> -				rc = unmap_and_move(get_new_page, put_new_page,
> +				continue;
> +
> +			rc = unmap_and_move(get_new_page, put_new_page,
>  						private, page, pass > 2, mode,
>  						reason, &ret_pages);
>  			/*
>  			 * The rules are:
> -			 *	Success: non hugetlb page will be freed, hugetlb
> -			 *		 page will be put back
> +			 *	Success: page will be freed
>  			 *	-EAGAIN: stay on the from list
>  			 *	-ENOMEM: stay on the from list
>  			 *	-ENOSYS: stay on the from list
> @@ -1468,7 +1524,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  						nr_thp_split++;
>  						break;
>  					}
> -				/* Hugetlb migration is unsupported */
>  				} else if (!no_subpage_counting) {
>  					nr_failed++;
>  				}
> -- 
> 2.35.1


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Best Regards,
Yan, Zi

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