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SubjectRe: [PATCH for 3.2] memcg: do not trap chargers with full callstack on OOM
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@cmpxchg.org>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, "cgroups mailinglist" <cgroups@vger.kernel.org>, "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>, righi.andrea@gmail.com
> > > >> CC: "Johannes Weiner" <hannes@cmpxchg.org>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, "cgroups mailinglist" <cgroups@vger.kernel.org>, "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>, righi.andrea@gmail.com
> > > >>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.

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.
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 :)


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