Re: [PATCH v2] mm/vmscan: don't try to reclaim freed folios

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 08, 2022 at 10:14:32PM +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 I/O.

Yes, but I asked how often this happened, and you said you didn't know.
Do you have any data?  I'm reluctant to make a function which is over
400 LOC already any longer.

> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
> v2:
>   use folio_ref_freeze to guard against race with GUP (fast). Many thanks
>   Matthew for pointing this out.
> ---
>  mm/vmscan.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 13d34d9593bb..547ae7ae6ab1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1610,13 +1610,19 @@ 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_ref_freeze(folio, 1)) {
> +			/* folio was freed from under us. So we are done. */
> +			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
>