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 <20221227002859.27740-3-ying.huang@intel.com>
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From:   Alistair Popple <apopple@nvidia.com>
To:     Huang Ying <ying.huang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Zi Yan <ziy@nvidia.com>,
        Yang Shi <shy828301@gmail.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        Bharata B Rao <bharata@amd.com>,
        haoxin <xhao@linux.alibaba.com>
Subject: Re: [PATCH 2/8] migrate_pages: separate hugetlb folios migration
Date:   Thu, 05 Jan 2023 15:13:25 +1100
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Huang Ying <ying.huang@intel.com> writes:

> 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.
>
> 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>
> Cc: Bharata B Rao <bharata@amd.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: haoxin <xhao@linux.alibaba.com>
> ---
>  mm/migrate.c | 114 ++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 99 insertions(+), 15 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ec9263a33d38..bdbe73fe2eb7 100644
> --- 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++) {
> +		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++;

This currently isn't relevant for -ENOMEM and I think it would be
clearer if it was dropped.

> +				stats->nr_failed_pages += nr_pages;

Makes sense not to continue migration with low memory, but shouldn't we
add the remaining unmigrated hugetlb folios to stats->nr_failed_pages as
well? Ie. don't we still have to continue the iteration to to find and
account for these?

> +				goto out;

Given this is the only use of the out label, and that there is a special
case for -ENOMEM there anyway I think it would be clearer to return
directly.

> +			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;
> +}
> +
>  /*
>   * migrate_pages - migrate the folios specified in a list, to the free folios
>   *		   supplied as the target for the page migration
> @@ -1437,7 +1518,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	int retry = 1;
>  	int large_retry = 1;
>  	int thp_retry = 1;
> -	int nr_failed = 0;
> +	int nr_failed;
>  	int nr_retry_pages = 0;
>  	int nr_large_failed = 0;
>  	int pass = 0;
> @@ -1454,6 +1535,12 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	trace_mm_migrate_pages_start(mode, reason);
>  
>  	memset(&stats, 0, sizeof(stats));
> +	rc = migrate_hugetlbs(from, get_new_page, put_new_page, private, mode, reason,
> +			      &stats, &ret_folios);
> +	if (rc < 0)
> +		goto out;
> +	nr_failed = rc;
> +
>  split_folio_migration:
>  	for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
>  		retry = 0;
> @@ -1462,30 +1549,28 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		nr_retry_pages = 0;
>  
>  		list_for_each_entry_safe(folio, folio2, from, lru) {
> +			if (folio_test_hugetlb(folio)) {

How do we hit this case? Shouldn't migrate_hugetlbs() have already moved
any hugetlb folios off the from list?

> +				list_move_tail(&folio->lru, &ret_folios);
> +				continue;
> +			}
> +
>  			/*
>  			 * Large folio statistics is based on the source large
>  			 * folio. Capture required information that might get
>  			 * lost during migration.
>  			 */
> -			is_large = folio_test_large(folio) && !folio_test_hugetlb(folio);
> +			is_large = folio_test_large(folio);
>  			is_thp = is_large && folio_test_pmd_mappable(folio);
>  			nr_pages = folio_nr_pages(folio);
> +
>  			cond_resched();
>  
> -			if (folio_test_hugetlb(folio))
> -				rc = unmap_and_move_huge_page(get_new_page,
> -						put_new_page, private,
> -						&folio->page, pass > 2, mode,
> -						reason,
> -						&ret_folios);
> -			else
> -				rc = unmap_and_move(get_new_page, put_new_page,
> -						private, folio, pass > 2, mode,
> -						reason, &ret_folios);
> +			rc = unmap_and_move(get_new_page, put_new_page,
> +					    private, folio, pass > 2, mode,
> +					    reason, &ret_folios);
>  			/*
>  			 * The rules are:
> -			 *	Success: non hugetlb folio will be freed, hugetlb
> -			 *		 folio will be put back
> +			 *	Success: folio will be freed
>  			 *	-EAGAIN: stay on the from list
>  			 *	-ENOMEM: stay on the from list
>  			 *	-ENOSYS: stay on the from list
> @@ -1512,7 +1597,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  						stats.nr_thp_split += is_thp;
>  						break;
>  					}
> -				/* Hugetlb migration is unsupported */
>  				} else if (!no_split_folio_counting) {
>  					nr_failed++;
>  				}