Re: [PATCH] mm: fix oom work when memory is under pressure

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

zhong jiang wrote:
> On 2016/9/14 19:29, Tetsuo Handa wrote:
> > On 2016/09/14 18:25, zhong jiang wrote:
> >> On 2016/9/14 16:52, Michal Hocko wrote:
> >>> On Wed 14-09-16 10:42:19, Michal Hocko wrote:
> >>>> [Let's CC Hugh]
> >>> now for real...
> >>>
> >>>> On Wed 14-09-16 15:13:50, zhong jiang wrote:
> >>>> [...]
> >>>>>   hi, Michal
> >>>>>
> >>>>>   Recently, I hit the same issue when run a OOM case of the LTP and ksm enable.
> >>>>>  
> >>>>> [  601.937145] Call trace:
> >>>>> [  601.939600] [<ffffffc000086a88>] __switch_to+0x74/0x8c
> >>>>> [  601.944760] [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc
> >>>>> [  601.950007] [<ffffffc000a1c09c>] schedule+0x3c/0x94
> >>>>> [  601.954907] [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350
> >>>>> [  601.961289] [<ffffffc000a1e32c>] down_write+0x64/0x80
> >>>>> [  601.966363] [<ffffffc00021f794>] __ksm_exit+0x90/0x19c
> >>>>> [  601.971523] [<ffffffc0000be650>] mmput+0x118/0x11c
> >>>>> [  601.976335] [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74
> >>>>> [  601.981321] [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4
> >>>>> [  601.986656] [<ffffffc0000d0f34>] get_signal+0x444/0x5e0
> >>>>> [  601.991904] [<ffffffc000089fcc>] do_signal+0x1d8/0x450
> >>>>> [  601.997065] [<ffffffc00008a35c>] do_notify_resume+0x70/0x78
> > Please be sure to include exact kernel version (e.g. "uname -r",
> > "cat /proc/version") when reporting.
> >
> > You are reporting a bug in 4.1-stable kernel, which was prone to
> > OOM livelock because the OOM reaper is not available.
> > ( http://lkml.kernel.org/r/57D8012F.7080508@huawei.com )
> >
> > I think we no longer can reproduce this bug using 4.8-rc6 (or linux-next),
> > but it will be a nice thing to backport __GFP_NORETRY patch to stable
> > kernels which do not have the OOM reaper.
>   No, OOM reaper can not  solve the issue completely , As had disscussed with Michal.
>   The conclusion is that we need come up with a better method to fix it.
> 
>    Thanks
>   zhongjiang

I still think we no longer can reproduce this bug using 4.8-rc6 (or linux-next).

As of 4.1-stable, this bug caused OOM livelock situation because TIF_MEMDIE was
cleared only after returning from mmput() from exit_mm() from do_exit(). Since
there is a TIF_MEMDIE thread waiting at mmput() exists, the OOM killer does not
select next OOM victim because oom_scan_process_thread() returns OOM_SCAN_ABORT,
although ksmd is waking up the OOM killer via a __GFP_FS allocation request.

As of 4.8-rc6, the OOM reaper cannot take mmap_sem for read at __oom_reap_task()
because of TIF_MEMDIE thread waiting at ksm_exit() from __mmput() from mmput()
 from exit_mm() from do_exit(). Thus, __oom_reap_task() returns false and
oom_reap_task() will emit "oom_reaper: unable to reap pid:%d (%s)\n" message.
Then, oom_reap_task() clears TIF_MEMDIE from that thread, which in turn
makes oom_scan_process_thread() not to return OOM_SCAN_ABORT because
atomic_read(&task->signal->oom_victims) becomes 0 due to exit_oom_victim()
by the OOM reaper. Then, the OOM killer selects next OOM victim because
ksmd is waking up the OOM killer via a __GFP_FS allocation request.

Thus, this bug will be completely solved (at the cost of selecting next
OOM victim) as of 4.8-rc6.

> >> Adding the __GFP_NORETRY,  the issue also can fixed.
> >> Therefore, we can assure that the case of LTP will leads to the endless looping.
> >>
> >> index d45a0a1..03fb67b 100644
> >> --- a/mm/ksm.c
> >> +++ b/mm/ksm.c
> >> @@ -283,7 +283,7 @@ static inline struct rmap_item *alloc_rmap_item(void)
> >>  {
> >>         struct rmap_item *rmap_item;
> >>
> >> -       rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
> >> +       rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL | __GFP_NORETRY);
> >>         if (rmap_item)
> >>                 ksm_rmap_items++;
> >>         return rmap_item;
> >

Your patch to add __GFP_NORETRY and __GFP_NOWARN is OK. But please explicitly state
that you hit this bug in 4.1-stable. Also, your trace is not only a hung task but also
an OOM livelock; the kernel as of 4.1-stable is silent when OOM livelock situation
occurred.

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