Re: [PATCH v4] mm/vmscan: more restrictive condition for retry in do_try_to_free_pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Mar 15, 2017 at 07:36:48PM +0800, Yisheng Xie wrote:
> @@ -100,6 +100,9 @@ struct scan_control {
>  	/* Can cgroups be reclaimed below their normal consumption range? */
>  	unsigned int may_thrash:1;
>  
> +	/* Did we have any memcg protected by the low limit */
> +	unsigned int memcg_low_protection:1;

These are both bad names. How about the following pair?

	/*
	 * Cgroups are not reclaimed below their configured memory.low,
	 * unless we threaten to OOM. If any cgroups are skipped due to
	 * memory.low and nothing was reclaimed, go back for memory.low.
	 */
	unsigned int memcg_low_skipped:1
	unsigned int memcg_low_reclaim:1;

> @@ -2557,6 +2560,8 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  			unsigned long scanned;
>  
>  			if (mem_cgroup_low(root, memcg)) {
> +				sc->memcg_low_protection = 1;
> +
>  				if (!sc->may_thrash)
>  					continue;

				if (!sc->memcg_low_reclaim) {
					sc->memcg_low_skipped = 1;
					continue;
				}

>  				mem_cgroup_events(memcg, MEMCG_LOW, 1);
> @@ -2808,7 +2813,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>  		return 1;
>  
>  	/* Untapped cgroup reserves?  Don't OOM, retry. */
> -	if (!sc->may_thrash) {
> +	if (sc->memcg_low_protection && !sc->may_thrash) {

	if (sc->memcg_low_skipped) {
		[...]
		sc->memcg_low_reclaim = 1;
		goto retry;
	}

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