I think this can be a fix.
maybe good to CC Oleg.
==
>From dff52fb35af0cf36486965d19ee79e04b59f1dc4 Mon Sep 17 00:00:00 2001
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Date: Fri, 10 Jun 2011 13:15:14 +0900
Subject: [PATCH] [BUGFIX] update mm->owner even if no next owner.
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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