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

[Dave Chinner <david@fromorbit.com>]

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@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.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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