Re: [PATCH] mm: mglru: clear the reference counter for rejected folios

[Baoquan He]

Hi Baolin,

On 09/07/26 at 11:25am, Baolin Wang wrote:
......snip...
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 40d3f1b48a74..42c0a09938ab 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c

Well, this seems to be based on Andrew's mm-new branch. I usually track
mm-unstable branch. Maybe the subject should be marked as below?
[PATCH mm-new] mm: mglru: clear the reference counter for rejected

> @@ -5021,10 +5021,11 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  		}
>  
>  		/* don't add rejected folios to the oldest generation */
> -		if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) {
> -			folio_set_lru_refs(folio, 0);
> +		if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
>  			folio_set_active(folio);
> -		}
> +
> +		/* See the comments on LRU_REFS_FLAGS */
> +		folio_set_lru_refs(folio, 0);

This looks like a great catch, while the code change could bring issue.

Because move_folios_to_lru() relies on folios' flags to decide their new
generation. You just cleared it before move_folios_to_lru(). This is no
problem for rejected folios that are determined to be put into the
oldest generation. But for those rejected folios that are determined to
be promoted, this could be wrong. E.g currently gen window is 4, and a
folio is referenced, lru_gen_folio_seq() decides its new gen as 1, which
is the 2nd oldest generation. While folio_set_lru_refs(folio, 0) clear
referenced bit, this causes it being put into the oldest generation in
move_folios_to_lru(), this is not expected.

Thanks
Baoquan
>  	}
>  
>  	move_folios_to_lru(&list);
> -- 
> 2.47.3
> 
>