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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 1/8] migrate_pages: organize stats with struct
 migrate_pages_stats
Date:   Thu, 05 Jan 2023 14:02:12 +1100
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Huang Ying <ying.huang@intel.com> writes:

> Define struct migrate_pages_stats to organize the various statistics
> in migrate_pages().  This makes it easier to collect and consume the
> statistics in multiple functions.  This will be needed in the
> following patches in the series.
>
> 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 | 58 +++++++++++++++++++++++++++++-----------------------
>  1 file changed, 32 insertions(+), 26 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a4d3fc65085f..ec9263a33d38 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1396,6 +1396,14 @@ static inline int try_split_folio(struct folio *folio, struct list_head *split_f
>  	return rc;
>  }
>  
> +struct migrate_pages_stats {
> +	int nr_succeeded;
> +	int nr_failed_pages;
> +	int nr_thp_succeeded;
> +	int nr_thp_failed;
> +	int nr_thp_split;

I think some brief comments in the code for what each stat is tracking
and their relationship to each other would be helpful (ie. does
nr_succeeded include thp subpages, etc). Or at least a reference to
where this is documented (ie. page_migration.rst) as I recall there has
been some confusion in the past that has lead to bugs.

Otherwise the patch looks good so:

Reviewed-by: Alistair Popple <apopple@nvidia.com>

> +};
> +
>  /*
>   * migrate_pages - migrate the folios specified in a list, to the free folios
>   *		   supplied as the target for the page migration
> @@ -1430,13 +1438,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	int large_retry = 1;
>  	int thp_retry = 1;
>  	int nr_failed = 0;
> -	int nr_failed_pages = 0;
>  	int nr_retry_pages = 0;
> -	int nr_succeeded = 0;
> -	int nr_thp_succeeded = 0;
>  	int nr_large_failed = 0;
> -	int nr_thp_failed = 0;
> -	int nr_thp_split = 0;
>  	int pass = 0;
>  	bool is_large = false;
>  	bool is_thp = false;
> @@ -1446,9 +1449,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	LIST_HEAD(split_folios);
>  	bool nosplit = (reason == MR_NUMA_MISPLACED);
>  	bool no_split_folio_counting = false;
> +	struct migrate_pages_stats stats;
>  
>  	trace_mm_migrate_pages_start(mode, reason);
>  
> +	memset(&stats, 0, sizeof(stats));
>  split_folio_migration:
>  	for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
>  		retry = 0;
> @@ -1502,9 +1507,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				/* Large folio migration is unsupported */
>  				if (is_large) {
>  					nr_large_failed++;
> -					nr_thp_failed += is_thp;
> +					stats.nr_thp_failed += is_thp;
>  					if (!try_split_folio(folio, &split_folios)) {
> -						nr_thp_split += is_thp;
> +						stats.nr_thp_split += is_thp;
>  						break;
>  					}
>  				/* Hugetlb migration is unsupported */
> @@ -1512,7 +1517,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  					nr_failed++;
>  				}
>  
> -				nr_failed_pages += nr_pages;
> +				stats.nr_failed_pages += nr_pages;
>  				list_move_tail(&folio->lru, &ret_folios);
>  				break;
>  			case -ENOMEM:
> @@ -1522,13 +1527,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				 */
>  				if (is_large) {
>  					nr_large_failed++;
> -					nr_thp_failed += is_thp;
> +					stats.nr_thp_failed += is_thp;
>  					/* Large folio NUMA faulting doesn't split to retry. */
>  					if (!nosplit) {
>  						int ret = try_split_folio(folio, &split_folios);
>  
>  						if (!ret) {
> -							nr_thp_split += is_thp;
> +							stats.nr_thp_split += is_thp;
>  							break;
>  						} else if (reason == MR_LONGTERM_PIN &&
>  							   ret == -EAGAIN) {
> @@ -1546,7 +1551,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  					nr_failed++;
>  				}
>  
> -				nr_failed_pages += nr_pages + nr_retry_pages;
> +				stats.nr_failed_pages += nr_pages + nr_retry_pages;
>  				/*
>  				 * There might be some split folios of fail-to-migrate large
>  				 * folios left in split_folios list. Move them back to migration
> @@ -1556,7 +1561,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				list_splice_init(&split_folios, from);
>  				/* nr_failed isn't updated for not used */
>  				nr_large_failed += large_retry;
> -				nr_thp_failed += thp_retry;
> +				stats.nr_thp_failed += thp_retry;
>  				goto out;
>  			case -EAGAIN:
>  				if (is_large) {
> @@ -1568,8 +1573,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				nr_retry_pages += nr_pages;
>  				break;
>  			case MIGRATEPAGE_SUCCESS:
> -				nr_succeeded += nr_pages;
> -				nr_thp_succeeded += is_thp;
> +				stats.nr_succeeded += nr_pages;
> +				stats.nr_thp_succeeded += is_thp;
>  				break;
>  			default:
>  				/*
> @@ -1580,20 +1585,20 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				 */
>  				if (is_large) {
>  					nr_large_failed++;
> -					nr_thp_failed += is_thp;
> +					stats.nr_thp_failed += is_thp;
>  				} else if (!no_split_folio_counting) {
>  					nr_failed++;
>  				}
>  
> -				nr_failed_pages += nr_pages;
> +				stats.nr_failed_pages += nr_pages;
>  				break;
>  			}
>  		}
>  	}
>  	nr_failed += retry;
>  	nr_large_failed += large_retry;
> -	nr_thp_failed += thp_retry;
> -	nr_failed_pages += nr_retry_pages;
> +	stats.nr_thp_failed += thp_retry;
> +	stats.nr_failed_pages += nr_retry_pages;
>  	/*
>  	 * Try to migrate split folios of fail-to-migrate large folios, no
>  	 * nr_failed counting in this round, since all split folios of a
> @@ -1626,16 +1631,17 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	if (list_empty(from))
>  		rc = 0;
>  
> -	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
> -	count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
> -	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
> -	count_vm_events(THP_MIGRATION_FAIL, nr_thp_failed);
> -	count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
> -	trace_mm_migrate_pages(nr_succeeded, nr_failed_pages, nr_thp_succeeded,
> -			       nr_thp_failed, nr_thp_split, mode, reason);
> +	count_vm_events(PGMIGRATE_SUCCESS, stats.nr_succeeded);
> +	count_vm_events(PGMIGRATE_FAIL, stats.nr_failed_pages);
> +	count_vm_events(THP_MIGRATION_SUCCESS, stats.nr_thp_succeeded);
> +	count_vm_events(THP_MIGRATION_FAIL, stats.nr_thp_failed);
> +	count_vm_events(THP_MIGRATION_SPLIT, stats.nr_thp_split);
> +	trace_mm_migrate_pages(stats.nr_succeeded, stats.nr_failed_pages,
> +			       stats.nr_thp_succeeded, stats.nr_thp_failed,
> +			       stats.nr_thp_split, mode, reason);
>  
>  	if (ret_succeeded)
> -		*ret_succeeded = nr_succeeded;
> +		*ret_succeeded = stats.nr_succeeded;
>  
>  	return rc;
>  }