Re: [PATCH for 3.2.34] memcg: do not trigger OOM from add_to_page_cache_locked

[Greg Thelen]

On Tue, Feb 05 2013, Michal Hocko wrote:

> On Tue 05-02-13 08:48:23, Greg Thelen wrote:
>> On Tue, Feb 05 2013, Michal Hocko wrote:
>> 
>> > On Tue 05-02-13 15:49:47, azurIt wrote:
>> > [...]
>> >> Just to be sure - am i supposed to apply this two patches?
>> >> http://watchdog.sk/lkml/patches/
>> >
>> > 5-memcg-fix-1.patch is not complete. It doesn't contain the folloup I
>> > mentioned in a follow up email. Here is the full patch:
>> > ---
>> > From f2bf8437d5b9bb38a95a432bf39f32c584955171 Mon Sep 17 00:00:00 2001
>> > From: Michal Hocko <mhocko@xxxxxxx>
>> > Date: Mon, 26 Nov 2012 11:47:57 +0100
>> > Subject: [PATCH] memcg: do not trigger OOM from add_to_page_cache_locked
>> >
>> > memcg oom killer might deadlock if the process which falls down to
>> > mem_cgroup_handle_oom holds a lock which prevents other task to
>> > terminate because it is blocked on the very same lock.
>> > This can happen when a write system call needs to allocate a page but
>> > the allocation hits the memcg hard limit and there is nothing to reclaim
>> > (e.g. there is no swap or swap limit is hit as well and all cache pages
>> > have been reclaimed already) and the process selected by memcg OOM
>> > killer is blocked on i_mutex on the same inode (e.g. truncate it).
>> >
>> > Process A
>> > [<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
>> >
>> > Process B
>> > [<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
>> 
>> It looks like grab_cache_page_write_begin() passes __GFP_FS into
>> __page_cache_alloc() and mem_cgroup_cache_charge().  Which makes me
>> think that this deadlock is also possible in the page allocator even
>> before getting to add_to_page_cache_lru.  no?
>
> I am not that familiar with VFS but i_mutex is a high level lock AFAIR
> and it shouldn't be called from the pageout path so __page_cache_alloc
> should be safe.

I wasn't clear, sorry.  My concern is not that pageout() grabs i_mutex.
My concern is that __page_cache_alloc() will invoke the oom killer and
select a victim which wants i_mutex.  This victim will deadlock because
the oom killer caller already holds i_mutex.  The wild accusation I am
making is that anyone who invokes the oom killer and waits on the victim
to die is essentially grabbing all of the locks that any of the oom
killer victims may grab (e.g. i_mutex).  To avoid deadlock the oom
killer can only be called is while holding no locks that the oom victim
demands.  I think some locks are grabbed in a way that allows the lock
request to fail if the task has a fatal signal pending, so they are
safe.  But any locks acquisitions that cannot fail (e.g. mutex_lock)
will deadlock with the oom killing process.  So the oom killing process
cannot hold any such locks which the victim will attempt to grab.
Hopefully I'm missing something.
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