Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file is low

[Michal Hocko]

Andrew,
it seems that this patch fallen through cracks. I am sorry if this was
due to my review feedback because it turned out that I missed the point
and later added my Acked-by. Sorry about that

On Mon 01-05-17 14:34:21, David Rientjes wrote:
> The purpose of the code that commit 623762517e23 ("revert 'mm: vmscan: do
> not swap anon pages just because free+file is low'") reintroduces is to
> prefer swapping anonymous memory rather than trashing the file lru.
> 
> If the anonymous inactive lru for the set of eligible zones is considered
> low, however, or the length of the list for the given reclaim priority
> does not allow for effective anonymous-only reclaiming, then avoid
> forcing SCAN_ANON.  Forcing SCAN_ANON will end up thrashing the small
> list and leave unreclaimed memory on the file lrus.
> 
> If the inactive list is insufficient, fallback to balanced reclaim so the
> file lru doesn't remain untouched.
> 
> Suggested-by: Minchan Kim <minchan@xxxxxxxxxx>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  to akpm: this issue has been possible since at least 3.15, so it's
>  probably not high priority for 4.12 but applies cleanly if it can sneak
>  in
> 
>  mm/vmscan.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2204,8 +2204,17 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
>  		}
>  
>  		if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
> -			scan_balance = SCAN_ANON;
> -			goto out;
> +			/*
> +			 * Force SCAN_ANON if there are enough inactive
> +			 * anonymous pages on the LRU in eligible zones.
> +			 * Otherwise, the small LRU gets thrashed.
> +			 */
> +			if (!inactive_list_is_low(lruvec, false, sc, false) &&
> +			    lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, sc->reclaim_idx)
> +					>> sc->priority) {
> +				scan_balance = SCAN_ANON;
> +				goto out;
> +			}
>  		}
>  	}
>  
> 
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-- 
Michal Hocko
SUSE Labs