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;

quoted

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