[Ocfs2-devel] [PATCH] fs/super.c: do not shrink fs slab during direct memory reclaim

[Junxiao Bi]

Hi Jiufei,

On 09/02/2014 05:03 PM, Xue jiufei wrote:
> Hi, Dave
> On 2014/9/2 7:51, Dave Chinner wrote:
>> On Fri, Aug 29, 2014 at 05:57:22PM +0800, Xue jiufei wrote:
>>> The patch trys to solve one deadlock problem caused by cluster
>>> fs, like ocfs2. And the problem may happen at least in the below
>>> situations:
>>> 1)Receiving a connect message from other nodes, node queues a
>>> work_struct o2net_listen_work.
>>> 2)o2net_wq processes this work and calls sock_alloc() to allocate
>>> memory for a new socket.
>>> 3)It would do direct memory reclaim when available memory is not
>>> enough and trigger the inode cleanup. That inode being cleaned up
>>> is happened to be ocfs2 inode, so call evict()->ocfs2_evict_inode()
>>> ->ocfs2_drop_lock()->dlmunlock()->o2net_send_message_vec(),
>>> and wait for the unlock response from master.
>>> 4)tcp layer received the response, call o2net_data_ready() and
>>> queue sc_rx_work, waiting o2net_wq to process this work.
>>> 5)o2net_wq is a single thread workqueue, it process the work one by
>>> one. Right now it is still doing o2net_listen_work and cannot handle
>>> sc_rx_work. so we deadlock.
>>>
>>> It is impossible to set GFP_NOFS for memory allocation in sock_alloc().
>>> So we use PF_FSTRANS to avoid the task reentering filesystem when
>>> available memory is not enough.
>>>
>>> Signed-off-by: joyce.xue <xuejiufei at huawei.com>
>>
>> For the second time: use memalloc_noio_save/memalloc_noio_restore.
>> And please put a great big comment in the code explaining why you
>> need to do this special thing with memory reclaim flags.
>>
>> Cheers,
>>
>> Dave.
>>
> Thanks for your reply. But I am afraid that memalloc_noio_save/
> memalloc_noio_restore can not solve my problem. __GFP_IO is cleared
> if PF_MEMALLOC_NOIO is set and can avoid doing IO in direct memory
> reclaim. However, __GFP_FS is still set that can not avoid pruning
> dcache and icache in memory allocation, resulting in the deadlock I
> described.

You can use PF_MEMALLOC_NOIO to replace PF_FSTRANS, set this flag in
ocfs2 and check it in sb shrinker.

Thanks,
Junxiao.
> 
> Thanks.
> XueJiufei
> 
> 
>