[Ocfs2-devel] A deadlock when system do not has sufficient memory

[Junxiao Bi <junxiao.bi@oracle.com>]

On 08/29/2014 03:22 PM, Xue jiufei wrote:
> On 2014/8/29 11:26, Junxiao Bi wrote:
>> On 08/28/2014 04:16 PM, Xue jiufei wrote:
>>> Hi Junxiao,
>>> On 2014/8/25 9:50, Junxiao Bi wrote:
>>>> Hi Jiufei,
>>>>
>>>> Maybe you can consider using PF_FSTRANS flag, set this flag before
>>>> allocating memory with GFP_KERNEL flag and unset after the allocation.
>>>> Checking this flag in ocfs2 when trying to free some pages during memory
>>>> direct reclaim. See an example from upstream commit
>>>> 5cf02d09b50b1ee1c2d536c9cf64af5a7d433f56 (nfs: skip commit in
>>>> releasepage if we're freeing memory for fs-related reasons) .
>>>>
>>>> Thanks,
>>>> Junxiao.
>>>>
>>> Thank you very much for your suggestion. But in our situation,
>>> o2net_wq is evicting inode during memory direct reclaim, which cannot
>>> return error or do nothing because vfs would destroy_inode after evict,
>>> but we haven't drop inode lock yet.
>> How about checking the flag in vfs like this? And you can set PF_FSTRANS
>> flag in o2net_wq context where GFP_NOFS flag can't be set.
>>
>>
>> commit 8d27fdec5ce234d2f02e4582d340d231396b92af
>> Author: Junxiao Bi <junxiao.bi@oracle.com>
>> Date:   Fri Aug 29 11:05:25 2014 +0800
>>
>>     super: stop shrinker for processes with PF_FSTRANS flag
>>
>>     For some cluster fs, like ocfs2, it may be impossible to
>>     set GFP_NOFS for some memory allocation, as the allocation
>>     is in network common code, like sock_alloc() and in this
>>     case, the shrinker will call back into the fs and cause
>>     deadlock when available memory is not enough.
>>
>>     Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
>>
>> diff --git a/fs/super.c b/fs/super.c
>> index b9a214d..c4a8dc1 100644
>> --- a/fs/super.c
>> +++ b/fs/super.c
>> @@ -71,6 +71,9 @@ static unsigned long super_cache_scan(struct shrinker
>> *shrink,
>>         if (!(sc->gfp_mask & __GFP_FS))
>>                 return SHRINK_STOP;
>>
>> +       if (current->flags & PF_FSTRANS)
>> +               return SHRINK_STOP;
>> +
>>         if (!grab_super_passive(sb))
>>                 return SHRINK_STOP;
>>
>>
>> Thanks,
>> Junxiao.
>>
> Yes, this patch can resolve our problem. Thanks a lot.
> Have you send this patch to fs-devel list?
No. May you send it with your ocfs2 fix? I think that is more convincing
with the ocfs2 deadlock case. I will monitor it if there is any concern
about it.

Thanks,
Junxiao.
>>>
>>> Thanks
>>> Xuejiufei
>>>
>>>> On 08/22/2014 04:30 PM, Xue jiufei wrote:
>>>>> On 2014/8/20 11:57, Xue jiufei wrote:
>>>>>> Hi all,
>>>>>> We found there may exist a deadlock when system has not sufficient
>>>>>> memory. Here's the situation:
>>>>>>             N1                                      N2
>>>>>>                                              send message to N1
>>>>>>       o2net_wq(kworker)
>>>>>> receiving message and call corresponding
>>>>>> handler to handle this message. It may 
>>>>>> need to alloc some memory(use GFP_NOFS or GFP_KERNEL).
>>>>>> but there's no sufficient memory, lower then
>>>>>> min watermark. So it wakeup kswapd to reclaim memory
>>>>>> and itself may also call
>>>>>> __alloc_pages_direct_reclaim(), trying to
>>>>>> free some pages.
>>>>>>
>>>>>> It tries to free ocfs2 inode
>>>>>> cache and calls ocfs2_drop_lock()->dlmunlock()
>>>>>> to drop inode lock, sending unlock message to master,
>>>>>> say N2. When reply comes, queue sc_rx_work and
>>>>>> wait o2net_wq to handle this work. however
>>>>>> o2net_wq is still handling last message, so can not 
>>>>>> process the reply message. It will wait
>>>>>> o2net_nsw_completed() in o2net_send_message_vec()
>>>>>> forever. 
>>>>>> Kswapd thread enconter the same situation.
>>>>>>
>>>>>>
>>>>>> So is there any advice to solve this deadlock?
>>>>>> And what is the probability that kmalloc return ENOMEM when use GFP_ATOMIC flag?
>>>>>>
>>>>>> Thanks.
>>>>>>
>>>>> To avoid this deadlock, we want to alloc memory with flag GFP_ATOMIC
>>>>> in all handlers and return ENOMEM to peer when failed. The peer will
>>>>> try to resend the message again, o2net_wq can handle other messages.
>>>>> However, it can not solve all problems. For example, if o2net_wq is
>>>>> processing sc_connect_work which would call sock_alloc_inode() to alloc
>>>>> socket_alloc with GFP_KERNEL flag when memory is insufficient and enter
>>>>> reclaim progress, it also trigger the deadlock. We can not change this
>>>>> alloc flag.
>>>>> We have no idea about it. Is there any better ideas. 
>>>>> Thanks very much.
>>>>> xuejiufei
>>>>>> _______________________________________________
>>>>>> Ocfs2-devel mailing list
>>>>>> Ocfs2-devel at oss.oracle.com
>>>>>> https://oss.oracle.com/mailman/listinfo/ocfs2-devel
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>>
>>>>
>>>> .
>>>>
>>>
>>>
>>
>> .
>>
> 
>