Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup is above protection

Michal Hocko
2020-04-30T14:57:42Z

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