Re: [PATCH 2/2] mm, oom: protect !costly allocations some more for !CONFIG_COMPACTION

[Hillf Danton]

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Joonsoo has reported that he is able to trigger OOM for !costly high
> order requests (heavy fork() workload close the OOM) with the new
> oom detection rework. This is because we rely only on should_reclaim_retry
> when the compaction is disabled and it only checks watermarks for the
> requested order and so we might trigger OOM when there is a lot of free
> memory.
> 
> It is not very clear what are the usual workloads when the compaction
> is disabled. Relying on high order allocations heavily without any
> mechanism to create those orders except for unbound amount of reclaim is
> certainly not a good idea.
> 
> To prevent from potential regressions let's help this configuration
> some. We have to sacrifice the determinsm though because there simply is
> none here possible. should_compact_retry implementation for
> !CONFIG_COMPACTION, which was empty so far, will do watermark check
> for order-0 on all eligible zones. This will cause retrying until either
> the reclaim cannot make any further progress or all the zones are
> depleted even for order-0 pages. This means that the number of retries
> is basically unbounded for !costly orders but that was the case before
> the rework as well so this shouldn't regress.
> 
> Reported-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  mm/page_alloc.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 620ec002aea2..7e2defbfe55b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3310,6 +3310,24 @@ should_compact_retry(struct alloc_context *ac, unsigned int order, int alloc_fla
>  		     enum migrate_mode *migrate_mode,
>  		     int compaction_retries)
>  {
> +	struct zone *zone;
> +	struct zoneref *z;
> +
> +	if (!order || order > PAGE_ALLOC_COSTLY_ORDER)
> +		return false;
> +
> +	/*
> +	 * There are setups with compaction disabled which would prefer to loop
> +	 * inside the allocator rather than hit the oom killer prematurely. Let's
> +	 * give them a good hope and keep retrying while the order-0 watermarks
> +	 * are OK.
> +	 */
> +	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx,
> +					ac->nodemask) {
> +		if(zone_watermark_ok(zone, 0, min_wmark_pages(zone),

s/if(zone_/if (zone_/
> +					ac_classzone_idx(ac), alloc_flags))
> +			return true;
> +	}
>  	return false;
>  }
>  #endif /* CONFIG_COMPACTION */
> --
> 2.8.1