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Date: Wed, 26 Nov 2008 11:14:47 +0900
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
To: balbir@linux.vnet.ibm.com
Cc: linux-mm@kvack.org, YAMAMOTO Takashi <yamamoto@valinux.co.jp>,
       Paul Menage <menage@google.com>, lizf@cn.fujitsu.com,
       linux-kernel@vger.kernel.org, Nick Piggin <nickpiggin@yahoo.com.au>,
       David Rientjes <rientjes@google.com>,
       Pavel Emelianov <xemul@openvz.org>,
       Dhaval Giani <dhaval@linux.vnet.ibm.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       nishimura@mxp.nes.nec.co.jp
Subject: Re: [mm] [PATCH 3/4] Memory cgroup hierarchical reclaim (v4)
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On Tue, 25 Nov 2008 20:31:25 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> Daisuke Nishimura wrote:
> > Hi.
> > 
> > Unfortunately, trying to hold cgroup_mutex at reclaim causes dead lock.
> > 
> > For example, when attaching a task to some cpuset directory(memory_migrate=on),
> > 
> >     cgroup_tasks_write (hold cgroup_mutex)
> >         attach_task_by_pid
> >             cgroup_attach_task
> >                 cpuset_attach
> >                     cpuset_migrate_mm
> >                         :
> >                         unmap_and_move
> >                             mem_cgroup_prepare_migration
> >                                 mem_cgroup_try_charge
> >                                     mem_cgroup_hierarchical_reclaim
> > 
> 
> Did lockdep complain about it?
> 
I haven't understood lockdep so well, but I got logs like this:

===
INFO: task move.sh:17710 blocked for more than 480 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
move.sh       D ffff88010e1c76c0     0 17710  17597
 ffff8800bd9edf00 0000000000000046 0000000000000000 0000000000000000
 ffff8803afbc0000 ffff8800bd9ee270 0000000e00000000 000000010a54459c
 ffffffffffffffff ffffffffffffffff ffffffffffffffff 7fffffffffffffff
Call Trace:
 [<ffffffff802ae9f0>] mem_cgroup_get_first_node+0x29/0x8a
 [<ffffffff804cb357>] mutex_lock_nested+0x180/0x2a2
 [<ffffffff802ae9f0>] mem_cgroup_get_first_node+0x29/0x8a
 [<ffffffff802ae9f0>] mem_cgroup_get_first_node+0x29/0x8a
 [<ffffffff802aed9c>] __mem_cgroup_try_charge+0x27a/0x2de
 [<ffffffff802afdfd>] mem_cgroup_prepare_migration+0x6c/0xa5
 [<ffffffff802ad97f>] migrate_pages+0x10c/0x4a0
 [<ffffffff802ad9c8>] migrate_pages+0x155/0x4a0
 [<ffffffff802a14cb>] new_node_page+0x0/0x2f
 [<ffffffff802a1adb>] check_range+0x300/0x325
 [<ffffffff802a2374>] do_migrate_pages+0x1a5/0x1f1
 [<ffffffff8026d272>] cpuset_migrate_mm+0x30/0x93
 [<ffffffff8026d29c>] cpuset_migrate_mm+0x5a/0x93
 [<ffffffff8026df41>] cpuset_attach+0x93/0xa6
 [<ffffffff8026ae1b>] cgroup_attach_task+0x395/0x3e1
 [<ffffffff8026af61>] cgroup_tasks_write+0xfa/0x11d
 [<ffffffff8026aea0>] cgroup_tasks_write+0x39/0x11d
 [<ffffffff8026b5aa>] cgroup_file_write+0xef/0x216
 [<ffffffff802b2968>] vfs_write+0xad/0x136
 [<ffffffff802b2dfe>] sys_write+0x45/0x6e
 [<ffffffff8020bdab>] system_call_fastpath+0x16/0x1b
INFO: lockdep is turned off.
===

And other processes trying to hold cgroup_mutex are also stuck.

> 1. We could probably move away from cgroup_mutex to a memory controller specific
> mutex.
> 2. We could give up cgroup_mutex before migrate_mm, since it seems like we'll
> hold the cgroup lock for long and holding it during reclaim will definitely be
> visible to users trying to create/delete nodes.
> 
> I prefer to do (2), I'll look at the code more closely
> 
I basically agree, but I think we should also consider mpol_rebind_mm.

mpol_rebind_mm, which can be called from cpuset_attach, does down_write(mm->mmap_sem),
which means down_write(mm->mmap_sem) can be called under cgroup_mutex.
OTOH, page fault path does down_read(mm->mmap_sem) and can call mem_cgroup_try_charge,
which means mutex_lock(cgroup_mutex) can be called under down_read(mm->mmap_sem).

> > I think similar problem can also happen when removing memcg's directory.
> > 
> 
> Why removing a directory? memcg (now) marks the directory as obsolete and we
> check for obsolete directories and get/put references.
> 
I don't think so.
    
    mem_cgroup_pre_destroy (make mem->obsolete = 1)
        mem_cgroup_force_empty(mem, **FALSE**)
            mem_cgroup_force_empty_list
                mem_cgroup_move_parent
                    __mem_cgroup_try_charge

hmm, but looking more closely, cgroup_call_pre_destroy is called
outside of cgroup_mutex, so this problem doesn't happen at rmdir probably.


Thanks,
Daisuke Nishimura.
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