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Date:   Fri, 19 Mar 2021 21:05:17 -0700
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Subject: Re: [PATCH 07/10] mm/vmscan: add helper for querying ability to age
 anonymous pages
From:   Greg Thelen <gthelen@google.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>,
        linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, Dave Hansen <dave.hansen@linux.intel.com>,
        rientjes@google.com, ying.huang@intel.com,
        dan.j.williams@intel.com, david@redhat.com, osalvador@suse.de
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Dave Hansen <dave.hansen@linux.intel.com> wrote:

> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Anonymous pages are kept on their own LRU(s).  These lists could
> theoretically always be scanned and maintained.  But, without swap,
> there is currently nothing the kernel can *do* with the results of a
> scanned, sorted LRU for anonymous pages.
>
> A check for '!total_swap_pages' currently serves as a valid check as
> to whether anonymous LRUs should be maintained.  However, another
> method will be added shortly: page demotion.
>
> Abstract out the 'total_swap_pages' checks into a helper, give it a
> logically significant name, and check for the possibility of page
> demotion.

Reviewed-by: Greg Thelen <gthelen@google.com>

> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: osalvador <osalvador@suse.de>
> ---
>
>  b/mm/vmscan.c |   28 +++++++++++++++++++++++++---
>  1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff -puN mm/vmscan.c~mm-vmscan-anon-can-be-aged mm/vmscan.c
> --- a/mm/vmscan.c~mm-vmscan-anon-can-be-aged	2021-03-04 15:35:58.935806422 -0800
> +++ b/mm/vmscan.c	2021-03-04 15:35:58.942806422 -0800
> @@ -2517,6 +2517,26 @@ out:
>  	}
>  }
>  
> +/*
> + * Anonymous LRU management is a waste if there is
> + * ultimately no way to reclaim the memory.
> + */
> +bool anon_should_be_aged(struct lruvec *lruvec)

Should this be static?

> +{
> +	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +
> +	/* Aging the anon LRU is valuable if swap is present: */
> +	if (total_swap_pages > 0)
> +		return true;
> +
> +	/* Also valuable if anon pages can be demoted: */
> +	if (next_demotion_node(pgdat->node_id) >= 0)
> +		return true;
> +
> +	/* No way to reclaim anon pages.  Should not age anon LRUs: */
> +	return false;
> +}
> +
>  static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  {
>  	unsigned long nr[NR_LRU_LISTS];
> @@ -2626,7 +2646,8 @@ static void shrink_lruvec(struct lruvec
>  	 * Even if we did not try to evict anon pages at all, we want to
>  	 * rebalance the anon lru active/inactive ratio.
>  	 */
> -	if (total_swap_pages && inactive_is_low(lruvec, LRU_INACTIVE_ANON))
> +	if (anon_should_be_aged(lruvec) &&
> +	    inactive_is_low(lruvec, LRU_INACTIVE_ANON))
>  		shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
>  				   sc, LRU_ACTIVE_ANON);
>  }
> @@ -3455,10 +3476,11 @@ static void age_active_anon(struct pglis
>  	struct mem_cgroup *memcg;
>  	struct lruvec *lruvec;
>  
> -	if (!total_swap_pages)
> +	lruvec = mem_cgroup_lruvec(NULL, pgdat);
> +
> +	if (!anon_should_be_aged(lruvec))
>  		return;
>  
> -	lruvec = mem_cgroup_lruvec(NULL, pgdat);
>  	if (!inactive_is_low(lruvec, LRU_INACTIVE_ANON))
>  		return;
>  
> _