Re: [PATCH v3 1/2] mm/migrate: correct nr_failed in migrate_pages_sync()

[Huang, Ying]

Zi Yan <zi.yan@xxxxxxxx> writes:

> From: Zi Yan <ziy@xxxxxxxxxx>
>
> nr_failed was missing the large folio splits from migrate_pages_batch()
> and can cause a mismatch between migrate_pages() return value and the
> number of not migrated pages, i.e., when the return value of
> migrate_pages() is 0, there are still pages left in the from page list.
> It will happen when a non-PMD THP large folio fails to migrate due to
> -ENOMEM and is split successfully but not all the split pages are not
> migrated, migrate_pages_batch() would return non-zero, but
> astats.nr_thp_split = 0. nr_failed would be 0 and returned to the caller
> of migrate_pages(), but the not migrated pages are left in the from page
> list without being added back to LRU lists.
>
> Fix it by adding a new nr_split counter for large folio splits and adding
> it to nr_failed in migrate_page_sync() after migrate_pages_batch() is
> done.
>
> Fixes: 2ef7dbb26990 ("migrate_pages: try migrate in batch asynchronously firstly")
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Huang Ying <ying.huang@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>

LGTM except a trivial issue below, not a big deal.  Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

> ---
>  mm/migrate.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c602bf6dec97..fc2e70b17704 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1495,6 +1495,7 @@ struct migrate_pages_stats {
>  	int nr_thp_succeeded;	/* THP migrated successfully */
>  	int nr_thp_failed;	/* THP failed to be migrated */
>  	int nr_thp_split;	/* THP split before migrating */
> +	int nr_split;	/* Large folio (include THP) split before migrating */
>  };
>  
>  /*
> @@ -1614,6 +1615,7 @@ static int migrate_pages_batch(struct list_head *from,
>  	int nr_retry_pages = 0;
>  	int pass = 0;
>  	bool is_thp = false;
> +	bool is_large = false;
>  	struct folio *folio, *folio2, *dst = NULL, *dst2;
>  	int rc, rc_saved = 0, nr_pages;
>  	LIST_HEAD(unmap_folios);
> @@ -1629,7 +1631,8 @@ static int migrate_pages_batch(struct list_head *from,
>  		nr_retry_pages = 0;
>  
>  		list_for_each_entry_safe(folio, folio2, from, lru) {
> -			is_thp = folio_test_large(folio) && folio_test_pmd_mappable(folio);
> +			is_large = folio_test_large(folio);
> +			is_thp = is_large && folio_test_pmd_mappable(folio);
>  			nr_pages = folio_nr_pages(folio);
>  
>  			cond_resched();
> @@ -1649,6 +1652,7 @@ static int migrate_pages_batch(struct list_head *from,
>  				stats->nr_thp_failed++;
>  				if (!try_split_folio(folio, split_folios)) {
>  					stats->nr_thp_split++;
> +					stats->nr_split++;
>  					continue;
>  				}
>  				stats->nr_failed_pages += nr_pages;
> @@ -1677,11 +1681,12 @@ static int migrate_pages_batch(struct list_head *from,
>  				nr_failed++;
>  				stats->nr_thp_failed += is_thp;
>  				/* Large folio NUMA faulting doesn't split to retry. */
> -				if (folio_test_large(folio) && !nosplit) {
> +				if (is_large && !nosplit) {
>  					int ret = try_split_folio(folio, split_folios);
>  
>  					if (!ret) {
>  						stats->nr_thp_split += is_thp;
> +						stats->nr_split += is_large;

We have tested is_large above, so, "++" is enough here.  Not a big deal,
only change it if you think it's necessary.

>  						break;
>  					} else if (reason == MR_LONGTERM_PIN &&
>  						   ret == -EAGAIN) {
> @@ -1827,6 +1832,7 @@ static int migrate_pages_sync(struct list_head *from, new_folio_t get_new_folio,
>  	stats->nr_succeeded += astats.nr_succeeded;
>  	stats->nr_thp_succeeded += astats.nr_thp_succeeded;
>  	stats->nr_thp_split += astats.nr_thp_split;
> +	stats->nr_split += astats.nr_split;
>  	if (rc < 0) {
>  		stats->nr_failed_pages += astats.nr_failed_pages;
>  		stats->nr_thp_failed += astats.nr_thp_failed;
> @@ -1834,7 +1840,11 @@ static int migrate_pages_sync(struct list_head *from, new_folio_t get_new_folio,
>  		return rc;
>  	}
>  	stats->nr_thp_failed += astats.nr_thp_split;
> -	nr_failed += astats.nr_thp_split;
> +	/*
> +	 * Do not count rc, as pages will be retried below.
> +	 * Count nr_split only, since it includes nr_thp_split.
> +	 */
> +	nr_failed += astats.nr_split;
>  	/*
>  	 * Fall back to migrate all failed folios one by one synchronously. All
>  	 * failed folios except split THPs will be retried, so their failure

--
Best Regards,
Huang, Ying