memcg: print cgroup information when system panics due to panic_on_oom

If kernel panics due to oom, caused by a cgroup reaching its limit, when
'compulsory panic_on_oom' is enabled, then we will only see that the OOM
happened because of "compulsory panic_on_oom is enabled" but this doesn't
tell the difference between mempolicy and memcg. And dumping system wide
information is plain wrong and more confusing. This patch provides the
information of the cgroup whose limit triggerred panic

Signed-off-by: Balasubramani Vivekanandan <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/oom.h

···
 extern void oom_zonelist_unlock(struct zonelist *zonelist, gfp_t gfp_flags);
 
 extern void check_panic_on_oom(enum oom_constraint constraint, gfp_t gfp_mask,
-			       int order, const nodemask_t *nodemask);
+			       int order, const nodemask_t *nodemask,
+			       struct mem_cgroup *memcg);
 
 extern enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
 		unsigned long totalpages, const nodemask_t *nodemask,

mm/memcontrol.c

···
 	struct mem_cgroup *iter;
 	unsigned int i;
 
-	if (!p)
-		return;
-
 	mutex_lock(&oom_info_lock);
 	rcu_read_lock();
 
-	pr_info("Task in ");
-	pr_cont_cgroup_path(task_cgroup(p, memory_cgrp_id));
-	pr_cont(" killed as a result of limit of ");
+	if (p) {
+		pr_info("Task in ");
+		pr_cont_cgroup_path(task_cgroup(p, memory_cgrp_id));
+		pr_cont(" killed as a result of limit of ");
+	} else {
+		pr_info("Memory limit reached of cgroup ");
+	}
+
 	pr_cont_cgroup_path(memcg->css.cgroup);
 	pr_cont("\n");
 
···
 		return;
 	}
 
-	check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, order, NULL);
+	check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, order, NULL, memcg);
 	totalpages = mem_cgroup_get_limit(memcg) ? : 1;
 	for_each_mem_cgroup_tree(iter, memcg) {
 		struct css_task_iter it;

mm/oom_kill.c

···
  * Determines whether the kernel must panic because of the panic_on_oom sysctl.
  */
 void check_panic_on_oom(enum oom_constraint constraint, gfp_t gfp_mask,
-			int order, const nodemask_t *nodemask)
+			int order, const nodemask_t *nodemask,
+			struct mem_cgroup *memcg)
 {
 	if (likely(!sysctl_panic_on_oom))
 		return;
···
 		if (constraint != CONSTRAINT_NONE)
 			return;
 	}
-	dump_header(NULL, gfp_mask, order, NULL, nodemask);
+	dump_header(NULL, gfp_mask, order, memcg, nodemask);
 	panic("Out of memory: %s panic_on_oom is enabled\n",
 		sysctl_panic_on_oom == 2 ? "compulsory" : "system-wide");
 }
···
 	constraint = constrained_alloc(zonelist, gfp_mask, nodemask,
 						&totalpages);
 	mpol_mask = (constraint == CONSTRAINT_MEMORY_POLICY) ? nodemask : NULL;
-	check_panic_on_oom(constraint, gfp_mask, order, mpol_mask);
+	check_panic_on_oom(constraint, gfp_mask, order, mpol_mask, NULL);
 
 	if (sysctl_oom_kill_allocating_task && current->mm &&
 	    !oom_unkillable_task(current, NULL, nodemask) &&