cpuset: fix to migrate mm correctly in a corner case

Before moving tasks out of empty cpusets, update_tasks_nodemask()
is called, which calls do_migrate_pages(xx, from, to). Then those
tasks are moved to an ancestor, and do_migrate_pages() is called
again.

The first time: from = node_to_be_offlined, to = empty.
The second time: from = empty, to = ancestor's nodemask.

so looks like no pages will be migrated.

Fix this by:

- Don't call update_tasks_nodemask() on empty cpusets.
- Pass cs->old_mems_allowed to do_migrate_pages().

v4: added comment in cpuset_hotplug_update_tasks() and rephased comment
    in cpuset_attach().
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cpuset.c

@@ -1563,9 +1563,18 @@ static void cpuset_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 		struct cpuset *mems_oldcs = effective_nodemask_cpuset(oldcs);
 
 		mpol_rebind_mm(mm, &cpuset_attach_nodemask_to);
-		if (is_memory_migrate(cs))
-			cpuset_migrate_mm(mm, &mems_oldcs->mems_allowed,
+
+		/*
+		 * old_mems_allowed is the same with mems_allowed here, except
+		 * if this task is being moved automatically due to hotplug.
+		 * In that case @mems_allowed has been updated and is empty,
+		 * so @old_mems_allowed is the right nodesets that we migrate
+		 * mm from.
+		 */
+		if (is_memory_migrate(cs)) {
+			cpuset_migrate_mm(mm, &mems_oldcs->old_mems_allowed,
 					  &cpuset_attach_nodemask_to);
+		}
 		mmput(mm);
 	}
 
@@ -2152,10 +2161,12 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs)
 
 	/*
 	 * If sane_behavior flag is set, we need to update tasks' cpumask
-	 * for empty cpuset to take on ancestor's cpumask.
+	 * for empty cpuset to take on ancestor's cpumask. Otherwise, don't
+	 * call update_tasks_cpumask() if the cpuset becomes empty, as
+	 * the tasks in it will be migrated to an ancestor.
 	 */
 	if ((sane && cpumask_empty(cs->cpus_allowed)) ||
-	    !cpumask_empty(&off_cpus))
+	    (!cpumask_empty(&off_cpus) && !cpumask_empty(cs->cpus_allowed)))
 		update_tasks_cpumask(cs, NULL);
 
 	mutex_lock(&callback_mutex);
@@ -2164,10 +2175,12 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs)
 
 	/*
 	 * If sane_behavior flag is set, we need to update tasks' nodemask
-	 * for empty cpuset to take on ancestor's nodemask.
+	 * for empty cpuset to take on ancestor's nodemask. Otherwise, don't
+	 * call update_tasks_nodemask() if the cpuset becomes empty, as
+	 * the tasks in it will be migratd to an ancestor.
 	 */
 	if ((sane && nodes_empty(cs->mems_allowed)) ||
-	    !nodes_empty(off_mems))
+	    (!nodes_empty(off_mems) && !nodes_empty(cs->mems_allowed)))
 		update_tasks_nodemask(cs, NULL);
 
 	is_empty = cpumask_empty(cs->cpus_allowed) ||