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

[Junxiao Bi]

On 09/03/2014 11:10 AM, Dave Chinner wrote:
> On Wed, Sep 03, 2014 at 09:38:31AM +0800, Junxiao Bi wrote:
>> 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.
> 
> No changes to the superblock shrinker, please. The flag should
> modify the gfp_mask in the struct shrink_control passed to the
> shrinker, just like the noio flag is used in the rest of the mm
> code.
__GFP_FS seemed imply __GFP_IO, can superblock shrinker check
!(sc->gfp_mask & __GFP_IO) and stop?

Thanks,
Junxiao.
> 
> Cheers,
> 
> Dave.
>