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

[Junxiao Bi]

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 at 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 at 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.

> 
> 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
>>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> Ocfs2-devel mailing list
>>> Ocfs2-devel at oss.oracle.com
>>> https://oss.oracle.com/mailman/listinfo/ocfs2-devel
>>>
>>
>> .
>>
> 
>