A deadlock when direct memory reclaim in network filesystem

A deadlock when direct memory reclaim in network filesystem

[Xue jiufei]

Hi all,
We found there may exist a deadlock during direct memory reclaim in
network filesystem.
Here's one example in ocfs2, maybe other network filesystems has
this problems too.

1)Receiving a connect message from other nodes, Node queued
o2net_listen_work.
2)o2net_wq processed this work and try to allocate memory for a
new socket.
3)Syetem has no more memory, it would do direct memory reclaim
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 response.
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 is is still doing o2net_listen_work and cannot handle
sc_rx_work. so we deadlock.

To avoid deadlock like this, caller should perform a GFP_NOFS
allocation attempt(see the comments of shrink_dcache_memory and
shrink_icache_memory).
However, in the situation I described above, it is impossible to
add GFP_NOFS flag unless we modify the socket create interface.

To fix this deadlock, we would not like to shrink inode and dentry
slab during direct memory reclaim. Kswapd would do this job for us.
So we want to force add __GFP_FS when call
__alloc_pages_direct_reclaim() in __alloc_pages_slowpath().
Is that OK or any better advice?

Thanks,
Xuejiufei

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Re: A deadlock when direct memory reclaim in network filesystem

[Dave Chinner]

On Thu, Aug 28, 2014 at 07:40:40PM +0800, Xue jiufei wrote:
> Hi all,
> We found there may exist a deadlock during direct memory reclaim in
> network filesystem.
> Here's one example in ocfs2, maybe other network filesystems has
> this problems too.
> 
> 1)Receiving a connect message from other nodes, Node queued
> o2net_listen_work.
> 2)o2net_wq processed this work and try to allocate memory for a
> new socket.
> 3)Syetem has no more memory, it would do direct memory reclaim
> 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 response.
> 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 is is still doing o2net_listen_work and cannot handle
> sc_rx_work. so we deadlock.
> 
> To avoid deadlock like this, caller should perform a GFP_NOFS
> allocation attempt(see the comments of shrink_dcache_memory and
> shrink_icache_memory).
> However, in the situation I described above, it is impossible to
> add GFP_NOFS flag unless we modify the socket create interface.
> 
> To fix this deadlock, we would not like to shrink inode and dentry
> slab during direct memory reclaim. Kswapd would do this job for us.
> So we want to force add __GFP_FS when call
> __alloc_pages_direct_reclaim() in __alloc_pages_slowpath().
> Is that OK or any better advice?

memalloc_noio_save/memalloc_noio_restore

-Dave.
-- 
Dave Chinner
david@fromorbit.com

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