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Date:   Thu, 30 Apr 2020 16:57:21 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Chris Down <chris@chrisdown.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Yafang Shao <laoar.shao@gmail.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup
 is above protection
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On Wed 29-04-20 12:56:27, Johannes Weiner wrote:
[...]
> I think to address this, we need a more comprehensive solution and
> introduce some form of serialization. I'm not sure yet how that would
> look like yet.

Yeah, that is what I've tried to express earlier and that is why I would
rather go with an uglier workaround for now and think about a more
robust effective values calculation on top.
 
> I'm still not sure it's worth having a somewhat ugly workaround in
> mem_cgroup_protection() to protect against half of the bug. If you
> think so, the full problem should at least be documented and marked
> XXX or something.

Yes, this makes sense to me. What about the following?
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 1b4150ff64be..50ffbc17cdd8 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -350,6 +350,42 @@ static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg,
 	if (mem_cgroup_disabled())
 		return 0;
 
+	/*
+	 * There is no reclaim protection applied to a targeted reclaim.
+	 * We are special casing this specific case here because
+	 * mem_cgroup_protected calculation is not robust enough to keep
+	 * the protection invariant for calculated effective values for
+	 * parallel reclaimers with different reclaim target. This is
+	 * especially a problem for tail memcgs (as they have pages on LRU)
+	 * which would want to have effective values 0 for targeted reclaim
+	 * but a different value for external reclaim.
+	 *
+	 * Example
+	 * Let's have global and A's reclaim in parallel:
+	 *  |
+	 *  A (low=2G, usage = 3G, max = 3G, children_low_usage = 1.5G)
+	 *  |\
+	 *  | C (low = 1G, usage = 2.5G)
+	 *  B (low = 1G, usage = 0.5G)
+	 *
+	 * For the global reclaim
+	 * A.elow = A.low
+	 * B.elow = min(B.usage, B.low) because children_low_usage <= A.elow
+	 * C.elow = min(C.usage, C.low)
+	 *
+	 * With the effective values resetting we have A reclaim
+	 * A.elow = 0
+	 * B.elow = B.low
+	 * C.elow = C.low
+	 *
+	 * If the global reclaim races with A's reclaim then
+	 * B.elow = C.elow = 0 because children_low_usage > A.elow)
+	 * is possible and reclaiming B would be violating the protection.
+	 *
+	 */
+	if (memcg == root)
+		return 0;
+
 	if (in_low_reclaim)
 		return READ_ONCE(memcg->memory.emin);
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 05b4ec2c6499..df88a22f09bc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6385,6 +6385,14 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 
 	if (!root)
 		root = root_mem_cgroup;
+
+	/*
+	 * Effective values of the reclaim targets are ignored so they
+	 * can be stale. Have a look at mem_cgroup_protection for more
+	 * details.
+	 * TODO: calculation should be more robust so that we do not need
+	 * that special casing.
+	 */
 	if (memcg == root)
 		return MEMCG_PROT_NONE;
 

> In practice, I doubt this matters all that much because limit reclaim
> and global reclaim tend to occur in complementary
> containerization/isolation strategies, not heavily simultaneously.

I would expect that as well but this is always hard to tell.

-- 
Michal Hocko
SUSE Labs