Date:	Mon, 5 Aug 2013 16:56:04 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	azurIt <azurit@...ox.sk>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, cgroups@...r.kernel.org, x86@...nel.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 7/7] mm: memcg: do not trap chargers with full callstack
 on OOM

On Mon, Aug 05, 2013 at 11:54:29AM +0200, Michal Hocko wrote:
> On Sat 03-08-13 13:00:00, Johannes Weiner wrote:
> > The memcg OOM handling is incredibly fragile and can deadlock.  When a
> > task fails to charge memory, it invokes the OOM killer and loops right
> > there in the charge code until it succeeds.  Comparably, any other
> > task that enters the charge path at this point will go to a waitqueue
> > right then and there and sleep until the OOM situation is resolved.
> > The problem is that these tasks may hold filesystem locks and the
> > mmap_sem; locks that the selected OOM victim may need to exit.
> > 
> > For example, in one reported case, the task invoking the OOM killer
> > was about to charge a page cache page during a write(), which holds
> > the i_mutex.  The OOM killer selected a task that was just entering
> > truncate() and trying to acquire the i_mutex:
> > 
> > OOM invoking task:
> > [<ffffffff8110a9c1>] mem_cgroup_handle_oom+0x241/0x3b0
> > [<ffffffff8110b5ab>] T.1146+0x5ab/0x5c0
> > [<ffffffff8110c22e>] mem_cgroup_cache_charge+0xbe/0xe0
> > [<ffffffff810ca28c>] add_to_page_cache_locked+0x4c/0x140
> > [<ffffffff810ca3a2>] add_to_page_cache_lru+0x22/0x50
> > [<ffffffff810ca45b>] grab_cache_page_write_begin+0x8b/0xe0
> > [<ffffffff81193a18>] ext3_write_begin+0x88/0x270
> > [<ffffffff810c8fc6>] generic_file_buffered_write+0x116/0x290
> > [<ffffffff810cb3cc>] __generic_file_aio_write+0x27c/0x480
> > [<ffffffff810cb646>] generic_file_aio_write+0x76/0xf0           # takes ->i_mutex
> > [<ffffffff8111156a>] do_sync_write+0xea/0x130
> > [<ffffffff81112183>] vfs_write+0xf3/0x1f0
> > [<ffffffff81112381>] sys_write+0x51/0x90
> > [<ffffffff815b5926>] system_call_fastpath+0x18/0x1d
> > [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > OOM kill victim:
> > [<ffffffff811109b8>] do_truncate+0x58/0xa0              # takes i_mutex
> > [<ffffffff81121c90>] do_last+0x250/0xa30
> > [<ffffffff81122547>] path_openat+0xd7/0x440
> > [<ffffffff811229c9>] do_filp_open+0x49/0xa0
> > [<ffffffff8110f7d6>] do_sys_open+0x106/0x240
> > [<ffffffff8110f950>] sys_open+0x20/0x30
> > [<ffffffff815b5926>] system_call_fastpath+0x18/0x1d
> > [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > The OOM handling task will retry the charge indefinitely while the OOM
> > killed task is not releasing any resources.
> > 
> > A similar scenario can happen when the kernel OOM killer for a memcg
> > is disabled and a userspace task is in charge of resolving OOM
> > situations.  In this case, ALL tasks that enter the OOM path will be
> > made to sleep on the OOM waitqueue and wait for userspace to free
> > resources or increase the group's limit.  But a userspace OOM handler
> > is prone to deadlock itself on the locks held by the waiting tasks.
> > For example one of the sleeping tasks may be stuck in a brk() call
> > with the mmap_sem held for writing but the userspace handler, in order
> > to pick an optimal victim, may need to read files from /proc/<pid>,
> > which tries to acquire the same mmap_sem for reading and deadlocks.
> > 
> > This patch changes the way tasks behave after detecting a memcg OOM
> > and makes sure nobody loops or sleeps with locks held:
> > 
> > 1. When OOMing in a user fault, invoke the OOM killer and restart the
> >    fault instead of looping on the charge attempt.  This way, the OOM
> >    victim can not get stuck on locks the looping task may hold.
> > 
> > 2. When OOMing in a user fault but somebody else is handling it
> >    (either the kernel OOM killer or a userspace handler), don't go to
> >    sleep in the charge context.  Instead, remember the OOMing memcg in
> >    the task struct and then fully unwind the page fault stack with
> >    -ENOMEM.  pagefault_out_of_memory() will then call back into the
> >    memcg code to check if the -ENOMEM came from the memcg, and then
> >    either put the task to sleep on the memcg's OOM waitqueue or just
> >    restart the fault.  The OOM victim can no longer get stuck on any
> >    lock a sleeping task may hold.
> > 
> > Reported-by: Reported-by: azurIt <azurit@...ox.sk>
> > Debugged-by: Michal Hocko <mhocko@...e.cz>
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> I was thinking whether we should add task_in_memcg_oom into return to
> the userspace path just in case but this should be OK for now and new
> users of mem_cgroup_enable_oom will be fought against hard.

Absolutely, I would have liked it to be at the lowest possible point
in the stack as well, but this seemed like a good trade off.  And I
expect the sites enabling and disabling memcg OOM killing to be fairly
static.

> Acked-by: Michal Hocko <mhocko@...e.cz>

Thanks!
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