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

[Balbir Singh]

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.


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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