linux-kernel - Re: [PATCH] mm/vmscan: don't try to reclaim freed folios

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Date:   Fri, 27 May 2022 16:02:25 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan: don't try to reclaim freed folios

On Fri, May 27, 2022 at 04:04:51PM +0800, Miaohe Lin wrote:
> If folios were freed from under us, there's no need to reclaim them. Skip
> these folios to save lots of cpu cycles and avoid possible unnecessary
> disk IO.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/vmscan.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f7d9a683e3a7..646dd1efad32 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1556,12 +1556,18 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  		folio = lru_to_folio(page_list);
>  		list_del(&folio->lru);
>  
> +		nr_pages = folio_nr_pages(folio);
> +		if (folio_ref_count(folio) == 1) {
> +			/* folio was freed from under us. So we are done. */
> +			WARN_ON(!folio_put_testzero(folio));

What?  No.  This can absolutely happen.  We have a refcount on the folio,
which means that any other thread can temporarily raise the refcount,
so this WARN_ON can trigger.  Also, we don't hold the folio locked,
or an extra reference, so nr_pages is unstable because it can be split.

> +			goto free_it;
> +		}
> +
>  		if (!folio_trylock(folio))
>  			goto keep;
>  
>  		VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
>  
> -		nr_pages = folio_nr_pages(folio);
>  
>  		/* Account the number of base pages */
>  		sc->nr_scanned += nr_pages;
> -- 
> 2.23.0
> 
>