Re: [PATCH 1/2] mm, memcg: allow OOM if no memcg is eligible during direct reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jun 11, 2014 at 10:00:23AM +0200, Michal Hocko wrote:
> If there is no memcg eligible for reclaim because all groups under the
> reclaimed hierarchy are within their guarantee then the global direct
> reclaim would end up in the endless loop because zones in the zonelists
> are not considered unreclaimable (as per all_unreclaimable) and so the
> OOM killer would never fire and direct reclaim would be triggered
> without no chance to reclaim anything.
> 
> This is not possible yet because reclaim falls back to ignore low_limit
> when nobody is eligible for reclaim. Following patch will allow to set
> the fallback mode to hard guarantee, though, so this is a preparatory
> patch.
> 
> Memcg reclaim doesn't suffer from this because the OOM killer is
> triggered after few unsuccessful attempts of the reclaim.
> 
> Fix this by checking the number of scanned pages which is obviously 0 if
> nobody is eligible and also check that the whole tree hierarchy is not
> eligible and tell OOM it can go ahead.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  mm/vmscan.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8041b0667673..99137aecd95f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2570,6 +2570,13 @@ out:
>  	if (aborted_reclaim)
>  		return 1;
>  
> +	/*
> +	 * If the target memcg is not eligible for reclaim then we have no option
> +	 * but OOM
> +	 */
> +	if (!sc->nr_scanned && mem_cgroup_all_within_guarantee(sc->target_mem_cgroup))
> +		return 0;

We can't just sprinkle `for each memcg in hierarchy` loops like this,
they can get really expensive.

It's pretty stupid to not have a return value on shrink_zone(), which
could easily indicate whether a zone was reclaimable, and instead have
another iteration over the same zonelist and the same memcg hierarchy
afterwards to figure out if shrink_zone() was successful or not.

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