Re: [PATCH 1/2] check the return value of soft_limit reclaim

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> In the global background reclaim, we do soft reclaim before scanning the
> per-zone LRU. However, the return value is ignored. This patch adds the logic
> where no per-zone reclaim happens if the soft reclaim raise the free pages
> above the zone's high_wmark.
> 
> I did notice a similar check exists but instead leaving a "gap" above the
> high_wmark(the code right after my change in vmscan.c). There are discussions
> on whether or not removing the "gap" which intends to balance pressures across
> zones over time. Without fully understand the logic behind, I didn't try to
> merge them into one, but instead adding the condition only for memcg users
> who care a lot on memory isolation.
> 
> Signed-off-by: Ying Han <yinghan@google.com>

Looks good to me. But this depend on "memcg soft limit" spec. To be honest,
I don't know this return value ignorance is intentional or not. So I think 
you need to get ack from memcg folks.


> ---
>  mm/vmscan.c |   16 +++++++++++++++-
>  1 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 060e4c1..e4601c5 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2320,6 +2320,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
>  	unsigned long total_scanned;
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
> +	unsigned long nr_soft_reclaimed;
>  	struct scan_control sc = {
>  		.gfp_mask = GFP_KERNEL,
>  		.may_unmap = 1,
> @@ -2413,7 +2414,20 @@ loop_again:
>  			 * Call soft limit reclaim before calling shrink_zone.
>  			 * For now we ignore the return value
>  			 */
> -			mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);
> +			nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
> +							order, sc.gfp_mask);
> +
> +			/*
> +			 * Check the watermark after the soft limit reclaim. If
> +			 * the free pages is above the watermark, no need to
> +			 * proceed to the zone reclaim.
> +			 */
> +			if (nr_soft_reclaimed && zone_watermark_ok_safe(zone,
> +					order, high_wmark_pages(zone),
> +					end_zone, 0)) {
> +				__inc_zone_state(zone, NR_SKIP_RECLAIM_GLOBAL);

NR_SKIP_RECLAIM_GLOBAL is defined by patch 2/2. please don't break bisectability.



> +				continue;
> +			}
>  
>  			/*
>  			 * We put equal pressure on every zone, unless
> -- 
> 1.7.3.1
> 



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