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
