From f047cecf2cfc9595b1f39c9aab383bb0682f5a53 Mon Sep 17 00:00:00 2001
From: Li Zefan <lizefan@huawei.com>
Date: Thu, 13 Jun 2013 15:11:44 +0800
Subject: [PATCH] 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>
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