Re: [PATCH for 3.2] memcg: do not trap chargers with full callstackon OOM

[Johannes Weiner]

On Tue, Jul 16, 2013 at 06:09:05PM +0200, Michal Hocko wrote:
> On Tue 16-07-13 11:35:44, Johannes Weiner wrote:
> > On Mon, Jul 15, 2013 at 06:00:06PM +0200, Michal Hocko wrote:
> > > On Mon 15-07-13 17:41:19, Michal Hocko wrote:
> > > > On Sun 14-07-13 01:51:12, azurIt wrote:
> > > > > > CC: "Johannes Weiner" <hannes@xxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, "cgroups mailinglist" <cgroups@xxxxxxxxxxxxxxx>, "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@xxxxxxxxxxxxxx>, righi.andrea@xxxxxxxxx
> > > > > >> CC: "Johannes Weiner" <hannes@xxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, "cgroups mailinglist" <cgroups@xxxxxxxxxxxxxxx>, "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@xxxxxxxxxxxxxx>, righi.andrea@xxxxxxxxx
> > > > > >>On Wed 10-07-13 18:25:06, azurIt wrote:
> > > > > >>> >> Now i realized that i forgot to remove UID from that cgroup before
> > > > > >>> >> trying to remove it, so cgroup cannot be removed anyway (we are using
> > > > > >>> >> third party cgroup called cgroup-uid from Andrea Righi, which is able
> > > > > >>> >> to associate all user's processes with target cgroup). Look here for
> > > > > >>> >> cgroup-uid patch:
> > > > > >>> >> https://www.develer.com/~arighi/linux/patches/cgroup-uid/cgroup-uid-v8.patch
> > > > > >>> >> 
> > > > > >>> >> ANYWAY, i'm 101% sure that 'tasks' file was empty and 'under_oom' was
> > > > > >>> >> permanently '1'.
> > > > > >>> >
> > > > > >>> >This is really strange. Could you post the whole diff against stable
> > > > > >>> >tree you are using (except for grsecurity stuff and the above cgroup-uid
> > > > > >>> >patch)?
> > > > > >>> 
> > > > > >>> 
> > > > > >>> Here are all patches which i applied to kernel 3.2.48 in my last test:
> > > > > >>> http://watchdog.sk/lkml/patches3/
> > > > > >>
> > > > > >>The two patches from Johannes seem correct.
> > > > > >>
> > > > > >>From a quick look even grsecurity patchset shouldn't interfere as it
> > > > > >>doesn't seem to put any code between handle_mm_fault and mm_fault_error
> > > > > >>and there also doesn't seem to be any new handle_mm_fault call sites.
> > > > > >>
> > > > > >>But I cannot tell there aren't other code paths which would lead to a
> > > > > >>memcg charge, thus oom, without proper FAULT_FLAG_KERNEL handling.
> > > > > >
> > > > > >
> > > > > >Michal,
> > > > > >
> > > > > >now i can definitely confirm that problem with unremovable cgroups
> > > > > >persists. What info do you need from me? I applied also your little
> > > > > >'WARN_ON' patch.
> > > > > 
> > > > > Ok, i think you want this:
> > > > > http://watchdog.sk/lkml/kern4.log
> > > > 
> > > > Jul 14 01:11:39 server01 kernel: [  593.589087] [ pid ]   uid  tgid total_vm      rss cpu oom_adj oom_score_adj name
> > > > Jul 14 01:11:39 server01 kernel: [  593.589451] [12021]  1333 12021   172027    64723   4       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.589647] [12030]  1333 12030   172030    64748   2       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.589836] [12031]  1333 12031   172030    64749   3       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.590025] [12032]  1333 12032   170619    63428   3       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.590213] [12033]  1333 12033   167934    60524   2       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.590401] [12034]  1333 12034   170747    63496   4       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.590588] [12035]  1333 12035   169659    62451   1       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.590776] [12036]  1333 12036   167614    60384   3       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.590984] [12037]  1333 12037   166342    58964   3       0             0 apache2
> > > > Jul 14 01:11:39 server01 kernel: [  593.591178] Memory cgroup out of memory: Kill process 12021 (apache2) score 847 or sacrifice child
> > > > Jul 14 01:11:39 server01 kernel: [  593.591370] Killed process 12021 (apache2) total-vm:688108kB, anon-rss:255472kB, file-rss:3420kB
> > > > Jul 14 01:11:41 server01 kernel: [  595.392920] ------------[ cut here ]------------
> > > > Jul 14 01:11:41 server01 kernel: [  595.393096] WARNING: at kernel/exit.c:888 do_exit+0x7d0/0x870()
> > > > Jul 14 01:11:41 server01 kernel: [  595.393256] Hardware name: S5000VSA
> > > > Jul 14 01:11:41 server01 kernel: [  595.393415] Pid: 12037, comm: apache2 Not tainted 3.2.48-grsec #1
> > > > Jul 14 01:11:41 server01 kernel: [  595.393577] Call Trace:
> > > > Jul 14 01:11:41 server01 kernel: [  595.393737]  [<ffffffff8105520a>] warn_slowpath_common+0x7a/0xb0
> > > > Jul 14 01:11:41 server01 kernel: [  595.393903]  [<ffffffff8105525a>] warn_slowpath_null+0x1a/0x20
> > > > Jul 14 01:11:41 server01 kernel: [  595.394068]  [<ffffffff81059c50>] do_exit+0x7d0/0x870
> > > > Jul 14 01:11:41 server01 kernel: [  595.394231]  [<ffffffff81050254>] ? thread_group_times+0x44/0xb0
> > > > Jul 14 01:11:41 server01 kernel: [  595.394392]  [<ffffffff81059d41>] do_group_exit+0x51/0xc0
> > > > Jul 14 01:11:41 server01 kernel: [  595.394551]  [<ffffffff81059dc7>] sys_exit_group+0x17/0x20
> > > > Jul 14 01:11:41 server01 kernel: [  595.394714]  [<ffffffff815caea6>] system_call_fastpath+0x18/0x1d
> > > > Jul 14 01:11:41 server01 kernel: [  595.394921] ---[ end trace 738570e688acf099 ]---
> > > > 
> > > > OK, so you had an OOM which has been handled by in-kernel oom handler
> > > > (it killed 12021) and 12037 was in the same group. The warning tells us
> > > > that it went through mem_cgroup_oom as well (otherwise it wouldn't have
> > > > memcg_oom.wait_on_memcg set and the warning wouldn't trigger) and then
> > > > it exited on the userspace request (by exit syscall).
> > > > 
> > > > I do not see any way how, this could happen though. If mem_cgroup_oom
> > > > is called then we always return CHARGE_NOMEM which turns into ENOMEM
> > > > returned by __mem_cgroup_try_charge (invoke_oom must have been set to
> > > > true).  So if nobody screwed the return value on the way up to page
> > > > fault handler then there is no way to escape.
> > > > 
> > > > I will check the code.
> > > 
> > > OK, I guess I found it:
> > > __do_fault
> > >   fault = filemap_fault
> > >   do_async_mmap_readahead
> > >     page_cache_async_readahead
> > >       ondemand_readahead
> > >         __do_page_cache_readahead
> > >           read_pages
> > >             readpages = ext3_readpages
> > >               mpage_readpages			# Doesn't propagate ENOMEM
> > >                add_to_page_cache_lru
> > >                  add_to_page_cache
> > >                    add_to_page_cache_locked
> > >                      mem_cgroup_cache_charge
> > > 
> > > So the read ahead most probably. Again! Duhhh. I will try to think
> > > about a fix for this. One obvious place is mpage_readpages but
> > > __do_page_cache_readahead ignores read_pages return value as well and
> > > page_cache_async_readahead, even worse, is just void and exported as
> > > such.
> > > 
> > > So this smells like a hard to fix bugger. One possible, and really ugly
> > > way would be calling mem_cgroup_oom_synchronize even if handle_mm_fault
> > > doesn't return VM_FAULT_ERROR, but that is a crude hack.
> > 
> > Ouch, good spot.
> > 
> > I don't think we need to handle an OOM from the readahead code.  If
> > readahead does not produce the desired page, we retry synchroneously
> > in page_cache_read() and handle the OOM properly.  We should not
> > signal an OOM for optional pages anyway.
> > 
> > So either we pass a flag from the readahead code down to
> > add_to_page_cache and mem_cgroup_cache_charge that tells the charge
> > code to ignore OOM conditions and do not set up an OOM context.
> 
> That was my previous attempt and it was sooo painful.
> 
> > Or we DO call mem_cgroup_oom_synchronize() from the read_cache_pages,
> > with an argument that makes it only clean up the context and not wait.
> 
> Yes, I was playing with this idea as well. I just do not like how
> fragile this is. We need some way to catch all possible places which
> might leak it.

I don't think this is necessary, but we could add a sanity check
in/near mem_cgroup_clear_userfault() that makes sure the OOM context
is only set up when an error is returned.

> > It would not be completely outlandish to place it there, since it's
> > right next to where an error from add_to_page_cache() is not further
> > propagated back through the fault stack.
> > 
> > I'm travelling right now, I'll send a patch when I get back
> > (Thursday).  Unless you beat me to it :)
> 
> I can cook something up but there is quite a big pile on my desk
> currently (as always :/).

No worries, I'll send an update.
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