Re: kernel warning: tried to kill an mm-less task!

[Li Zefan <lizf@cn.fujitsu.com>]

CC: Oleg Nesterov <oleg@tv-sign.ru>

Li Zefan wrote:
> Balbir Singh wrote:
>> Li Zefan wrote:
>>> When I ran the same test program I described in a previous patch,
>>> I got the following warning:
>>>
>>> WARNING: at mm/oom_kill.c:320 __oom_kill_task+0x6d/0x101()
>>> Modules linked in: 
>>>
>>> Pid: 3856, comm: a.out Not tainted 2.6.25-rc8-mm2 #37
>>>  [<ffffffff80243941>] warn_on_slowpath+0x64/0xa2
>>>  [<ffffffff80244e16>] printk+0x5e/0x7b
>>>  [<ffffffff8022b096>] page_count+0x25/0x49
>>>  [<ffffffff8022b2cd>] show_mem+0x125/0x15a
>>>  [<ffffffff8028f00f>] __oom_kill_task+0x6d/0x101
>>>  [<ffffffff8028f319>] oom_kill_process+0x16c/0x22e
>>>  [<ffffffff8028f72c>] select_bad_process+0xb0/0x122
>>>  [<ffffffff8028f8d3>] mem_cgroup_out_of_memory+0x65/0x8a
>>>  [<ffffffff802bee84>] mem_cgroup_charge_common+0xf8/0x215
>>>  [<ffffffff802a14ac>] handle_mm_fault+0x216/0x6c8
>>>  [<ffffffff8029ebca>] follow_page+0x191/0x27d
>>>  [<ffffffff80234155>] need_resched+0x31/0x4f
>>>  [<ffffffff802a1c53>] get_user_pages+0x2f5/0x3eb
>>>  [<ffffffff802a1f64>] make_pages_present+0x9e/0xca
>>>  [<ffffffff802a51fc>] mmap_region+0x38c/0x452
>>>  [<ffffffff802119c4>] arch_get_unmapped_area_topdown+0x1bf/0x2a7
>>>  [<ffffffff802a5971>] do_mmap_pgoff+0x321/0x39b
>>>  [<ffffffff805037ee>] _cond_resched+0x1c/0x5f
>>>  [<ffffffff80211715>] sys_mmap+0xf5/0x138
>>>  [<ffffffff8020c6d2>] tracesys+0xd5/0xda
>>> ---[ end trace fe959fb2f0473e7c ]---
>>> tried to kill an mm-less task!
>>>
>>> This showed up several times in some seconds, but then didn't appear
>>> any more. And it's reproducable in a x86_64 box, but doesn't happen
>>> in a x86_32 one.
>>>
>>> And this happens both with and without the oops fixing.
>>>
>> Could we get some more details on which task was chosen to be killed? It will be
>> nice to see the task flags as well to see if PF_EXITING is set.
>>
>> oom_kill_task() has a big WARNING in the comment
>>
>>         /* WARNING: mm may not be dereferenced since we did not obtain its
>>          * value from get_task_mm(p).  This is OK since all we need to do is
>>          * compare mm to q->mm below.
>>
>>
>> I want to see the flags to see if
>>
>> PF_BORROWED_MM or PF_EXIT* is set.
>>
>>
> 
> OK, I'll try.
> 
> --

I Added 2 printk()s:

 static void __oom_kill_task(struct task_struct *p, int verbose)
 {
+       printk(KERN_WARNING "pid = %d, flags = %x\n", p->pid, p->flags);
+
        if (is_global_init(p)) {
                WARN_ON(1);
                printk(KERN_WARNING "tried to kill init!\n");
@@ -319,6 +320,7 @@ static void __oom_kill_task(struct task_struct *p, int verbo

        if (!p->mm) {
                WARN_ON(1);
+               printk(KERN_WARNING "pid = %d, flags = %x\n", p->pid, p->flags);
                printk(KERN_WARNING "tried to kill an mm-less task!\n");
                return;
        }

got this:

pid = 3817, flags = 400140
------------[ cut here ]------------
WARNING: at mm/oom_kill.c:322 __oom_kill_task+0x74/0xf1()
...
---[ end trace bb92f2fd8fe6c7c5 ]---
pid = 3817, flags = 400144
tried to kill an mm-less task!

So PF_EXITING may be set during the call of oom_kill_task(), and I notice
the comment in oom_kill_task():

	 * Furthermore, even if mm contains a non-NULL value, p->mm may
	 * change to NULL at any time since we do not hold task_lock(p).
	 * However, this is of no concern to us.

Is this warning just harmless so that we can just ignore it ?