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Subject: Re: Sleeping BUG in khugepaged for i586
To: Andrew Morton <akpm@linux-foundation.org>,
        Larry Finger <Larry.Finger@lwfinger.net>
Cc: LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
References: <968ae9a9-5345-18ca-c7ce-d9beaf9f43b6@lwfinger.net>
 <20170605144401.5a7e62887b476f0732560fa0@linux-foundation.org>
From: Vlastimil Babka <vbabka@suse.cz>
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Date: Tue, 6 Jun 2017 16:02:10 +0200
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On 06/05/2017 11:44 PM, Andrew Morton wrote:
> On Sat, 3 Jun 2017 14:24:26 -0500 Larry Finger <Larry.Finger@lwfinger.net> wrote:
> 
>> I recently turned on locking diagnostics for a Dell Latitude D600 laptop, which 
>> requires a 32-bit kernel. In the log I found the following:
>>
>> BUG: sleeping function called from invalid context at mm/khugepaged.c:655
>> in_atomic(): 1, irqs_disabled(): 0, pid: 20, name: khugepaged
>> 1 lock held by khugepaged/20:
>>   #0:  (&mm->mmap_sem){++++++}, at: [<c03d6609>] 
>> collapse_huge_page.isra.47+0x439/0x1240
>> CPU: 0 PID: 20 Comm: khugepaged Tainted: G        W 

W means thre was WARN earler. Could be related... Got logs?

>> 4.12.0-rc1-wl-12125-g952a068 #80

What is "wl-12125-g952a068"? What patches on top of mainline?

>> Hardware name: Dell Computer Corporation Latitude D600 
>> /03U652, BIOS A05 05/29/2003
>> Call Trace:
>>   dump_stack+0x76/0xb2
>>   ___might_sleep+0x174/0x230
>>   collapse_huge_page.isra.47+0xacf/0x1240
>>   khugepaged_scan_mm_slot+0x41e/0xc00
>>   ? _raw_spin_lock+0x46/0x50
>>   khugepaged+0x277/0x4f0
>>   ? prepare_to_wait_event+0xe0/0xe0
>>   kthread+0xeb/0x120
>>   ? khugepaged_scan_mm_slot+0xc00/0xc00
>>   ? kthread_create_on_node+0x30/0x30
>>   ret_from_fork+0x21/0x30
>>
>> I have no idea when this problem was introduced. Of course, I will test any 
>> proposed fixes.
>>
> 
> Odd.  There's nothing wrong with cond_resched() while holding mmap_sem.
> It looks like khugepaged forgot to do a spin_unlock somewhere and we
> leaked a preempt_count.

Hmm I'd expect such spin lock to be reported together with mmap_sem in
the debugging "locks held" message?

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