[PATCH] [BUGFIX] update mm->owner even if no next owner.

[KAMEZAWA Hiroyuki]

A panic is reported.

> Call Trace:
> Â [<ffffffff81139792>] mem_cgroup_from_task+0x15/0x17
> Â [<ffffffff8113a75a>] __mem_cgroup_try_charge+0x148/0x4b4
> Â [<ffffffff810493f3>] ? need_resched+0x23/0x2d
> Â [<ffffffff814cbf43>] ? preempt_schedule+0x46/0x4f
> Â [<ffffffff8113afe8>] mem_cgroup_charge_common+0x9a/0xce
> Â [<ffffffff8113b6d1>] mem_cgroup_newpage_charge+0x5d/0x5f
> Â [<ffffffff81134024>] khugepaged+0x5da/0xfaf
> Â [<ffffffff81078ea0>] ? __init_waitqueue_head+0x4b/0x4b
> Â [<ffffffff81133a4a>] ? add_mm_counter.constprop.5+0x13/0x13
> Â [<ffffffff81078625>] kthread+0xa8/0xb0
> Â [<ffffffff814d13e8>] ? sub_preempt_count+0xa1/0xb4
> Â [<ffffffff814d5664>] kernel_thread_helper+0x4/0x10
> Â [<ffffffff814ce858>] ? retint_restore_args+0x13/0x13
> Â [<ffffffff8107857d>] ? __init_kthread_worker+0x5a/0x5a

The code is.
>         return container_of(task_subsys_state(p, mem_cgroup_subsys_id),
>                                 struct mem_cgroup, css);


What happens here is accssing a freed task struct "p" from mm->owner.
So, it's doubtful that mm->owner points to freed task struct.

At thread exit, we need to handle mm->owner. If exitting-thread == mm->owner,
we modify mm->owner to points to other exisiting task. But, we do not update
mm->owner when there are no more threads. But if a kernel thread, like khugepaged,
picks up a mm_struct without updating mm->users, there is a trouble.

When mm_users shows that the task is the last task belongs to mm.
mm->owner is not updated and remained to point to the task. So, in this case,
mm->owner points to a not exisiting task.  This was good because if there
are no thread, no charge happens in old days. But now, we have ksm and
khugepaged.

rcu_read_lock() used in memcg is of no use because mm->owner can be
freed before we take rcu_read_lock.
Then, mm->owner should be cleared if there are no next owner.

Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reported-by: Dave Jones <davej@xxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 kernel/exit.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 20a4064..dbc3736 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -582,8 +582,10 @@ void mm_update_next_owner(struct mm_struct *mm)
 	struct task_struct *c, *g, *p = current;
 
 retry:
-	if (!mm_need_new_owner(mm, p))
+	if (!mm_need_new_owner(mm, p)) {
+		rcu_assign_pointer(mm->owner, NULL);
 		return;
+	}
 
 	read_lock(&tasklist_lock);
 	/*
@@ -617,7 +619,7 @@ retry:
 	 * most likely racing with swapoff (try_to_unuse()) or /proc or
 	 * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
 	 */
-	mm->owner = NULL;
+	rcu_assign_pointer(mm->owner, NULL);
 	return;
 
 assign_new_owner:
-- 
1.7.4.1


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