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 12:21:24PM +0800, Junxiao Bi wrote:
> 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@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,

Now you are starting to understand. Check what GFP_NOIO actually
means, then tell me why memalloc_noio_flags() is not fully correct,
needs fixing, and needs to be applied to all of reclaim.

Hint: there's a heirarchy involved....

> can superblock shrinker check
> !(sc->gfp_mask & __GFP_IO) and stop?

No. Go back and read what I said about the initial setting of
sc->gfp_mask.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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