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Date:   Thu, 5 Apr 2018 15:45:26 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Tejun Heo <tj@kernel.org>, kernel-team@fb.com,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/4] mm: treat memory.low value inclusive
Message-ID: <20180405194526.GC27918@cmpxchg.org>
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 <20180405185921.4942-3-guro@fb.com>
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On Thu, Apr 05, 2018 at 07:59:20PM +0100, Roman Gushchin wrote:
> If memcg's usage is equal to the memory.low value, avoid reclaiming
> from this cgroup while there is a surplus of reclaimable memory.
> 
> This sounds more logical and also matches memory.high and memory.max
> behavior: both are inclusive.

I was trying to figure out why we did it this way in the first place
and found this patch:

commit 4e54dede38b45052a941bcf709f7d29f2e18174d
Author: Michal Hocko <mhocko@suse.cz>
Date:   Fri Feb 27 15:51:46 2015 -0800

    memcg: fix low limit calculation
    
    A memcg is considered low limited even when the current usage is equal to
    the low limit.  This leads to interesting side effects e.g.
    groups/hierarchies with no memory accounted are considered protected and
    so the reclaim will emit MEMCG_LOW event when encountering them.
    
    Another and much bigger issue was reported by Joonsoo Kim.  He has hit a
    NULL ptr dereference with the legacy cgroup API which even doesn't have
    low limit exposed.  The limit is 0 by default but the initial check fails
    for memcg with 0 consumption and parent_mem_cgroup() would return NULL if
    use_hierarchy is 0 and so page_counter_read would try to dereference NULL.
    
    I suppose that the current implementation is just an overlook because the
    documentation in Documentation/cgroups/unified-hierarchy.txt says:
    
      "The memory.low boundary on the other hand is a top-down allocated
      reserve.  A cgroup enjoys reclaim protection when it and all its
      ancestors are below their low boundaries"
    
    Fix the usage and the low limit comparision in mem_cgroup_low accordingly.
    
> @@ -5709,7 +5709,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
>  	elow = min(elow, parent_elow * low_usage / siblings_low_usage);
>  exit:
>  	memcg->memory.elow = elow;
> -	return usage < elow;
> +	return usage <= elow;

So I think this needs to be usage && usage <= elow to not emit
MEMCG_LOW events in case usage == elow == 0.