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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Zi Yan <zi.yan@sent.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Zi Yan <ziy@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>
Subject: Re: [PATCH 1/1] mm/migrate: correct nr_failed in migrate_pages_sync()
References: <20231009203231.1715845-1-zi.yan@sent.com>
        <20231009203231.1715845-2-zi.yan@sent.com>
Date:   Tue, 10 Oct 2023 11:36:34 +0800
In-Reply-To: <20231009203231.1715845-2-zi.yan@sent.com> (Zi Yan's message of
        "Mon, 9 Oct 2023 16:32:31 -0400")
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Zi Yan <zi.yan@sent.com> writes:

> From: Zi Yan <ziy@nvidia.com>
>
> 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@nvidia.com>
> ---
>  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