There are two cases when effective memory guarantee calculation is mistakenly skipped: 1) If memcg is a child of the root cgroup, and the root cgroup is not root_mem_cgroup (in other words, if the reclaim is targeted). Top-level memory cgroups are handled specially in mem_cgroup_protected(), because the root memory cgroup doesn't have memory guarantee and can't limit its children guarantees. So, all effective guarantee calculation is skipped. But in case of targeted reclaim things are different: cgroups, which parent exceeded its memory limit aren't special. 2) If memcg has no charged memory (memory usage is 0). In this case mem_cgroup_protected() always returns MEMCG_PROT_NONE, which is correct and prevents to generate fake memory low events for empty cgroups. But skipping memory emin/elow calculation is wrong: if there is no global memory pressure there might be no good chance again, so we can end up with effective guarantees set to 0 without any reason. Link: http://lkml.kernel.org/r/20180522132528.23769-2-guro@xxxxxx Signed-off-by: Roman Gushchin <guro@xxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Greg Thelen <gthelen@xxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Shuah Khan <shuah@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 485df6f63d26..3220c992ee26 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5477,15 +5477,10 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, if (mem_cgroup_disabled()) return MEMCG_PROT_NONE; - if (!root) - root = root_mem_cgroup; - if (memcg == root) + if (memcg == root_mem_cgroup) return MEMCG_PROT_NONE; usage = page_counter_read(&memcg->memory); - if (!usage) - return MEMCG_PROT_NONE; - emin = memcg->memory.min; elow = memcg->memory.low; @@ -5494,7 +5489,7 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, if (!parent) return MEMCG_PROT_NONE; - if (parent == root) + if (parent == root_mem_cgroup) goto exit; parent_emin = READ_ONCE(parent->memory.emin); @@ -5529,6 +5524,12 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, memcg->memory.emin = emin; memcg->memory.elow = elow; + if (root && memcg == root) + return MEMCG_PROT_NONE; + + if (!usage) + return MEMCG_PROT_NONE; + if (usage <= emin) return MEMCG_PROT_MIN; else if (usage <= elow) -- 2.14.4
