Re: [PATCH 1/1] 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 rc value 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.
>
> Fixes: 2ef7dbb26990 ("migrate_pages: try migrate in batch asynchronously firstly")
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c602bf6dec97..5348827bd958 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1834,7 +1834,7 @@ 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;
> +	nr_failed += rc + astats.nr_thp_split;
>  	/*
>  	 * Fall back to migrate all failed folios one by one synchronously. All
>  	 * failed folios except split THPs will be retried, so their failure

I don't think this is a correct fix.  The failed folios will be retried
in the following synchronous migration below.

To fix the issue, we should track nr_split for all large folios (not
only THP), then use

        nr_failed += astats.nr_split;

--
Best Regards,
Huang, Ying