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

[Michal Hocko <mhocko@suse.cz>]

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@suse.cz>
> > 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.
-- 
Michal Hocko
SUSE Labs

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