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Subject: Re: WARNING in try_charge
To:     Michal Hocko <mhocko@kernel.org>
Cc:     syzbot <syzbot+bab151e82a4e973fa325@syzkaller.appspotmail.com>,
        cgroups@vger.kernel.org, dvyukov@google.com, hannes@cmpxchg.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com, vdavydov.dev@gmail.com
References: <fc6e173e-8bda-269f-d44f-1c5f5215beac@I-love.SAKURA.ne.jp>
 <0000000000006350880572c61e62@google.com>
 <20180806174410.GB10003@dhcp22.suse.cz>
 <20180806175627.GC10003@dhcp22.suse.cz>
 <078bde8d-b1b5-f5ad-ed23-0cd94b579f9e@i-love.sakura.ne.jp>
 <20180806203437.GK10003@dhcp22.suse.cz>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <3cf8f630-73b7-20d4-8ad1-bb1c657ee30d@i-love.sakura.ne.jp>
Date:   Tue, 7 Aug 2018 05:46:04 +0900
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On 2018/08/07 5:34, Michal Hocko wrote:
> On Tue 07-08-18 05:26:23, Tetsuo Handa wrote:
>> On 2018/08/07 2:56, Michal Hocko wrote:
>>> So the oom victim indeed passed the above force path after the oom
>>> invocation. But later on hit the page fault path and that behaved
>>> differently and for some reason the force path hasn't triggered. I am
>>> wondering how could we hit the page fault path in the first place. The
>>> task is already killed! So what the hell is going on here.
>>>
>>> I must be missing something obvious here.
>>>
>> YOU ARE OBVIOUSLY MISSING MY MAIL!
>>
>> I already said this is "mm, oom: task_will_free_mem(current) should ignore MMF_OOM_SKIP for once."
>> problem which you are refusing at https://www.spinics.net/lists/linux-mm/msg133774.html .
>> And you again ignored my mail. Very sad...
> 
> Your suggestion simply didn't make much sense. There is nothing like
> first check is different from the rest.
> 

I don't think your patch is appropriate. It avoids hitting WARN(1) but does not avoid
unnecessary killing of OOM victims.

If you look at https://syzkaller.appspot.com/text?tag=CrashLog&x=15a1c770400000 , you will
notice that both 23766 and 23767 are killed due to task_will_free_mem(current) == false.
This is "unnecessary killing of additional processes".

[  365.869417] syz-executor2 invoked oom-killer: gfp_mask=0x6000c0(GFP_KERNEL), order=0, oom_score_adj=0
[  365.878899] CPU: 0 PID: 23767 Comm: syz-executor2 Not tainted 4.18.0-rc6-next-20180725+ #18
(...snipped...)
[  366.487490] Tasks state (memory values in pages):
[  366.492349] [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
[  366.501237] [  23766]     0 23766    17620     8221   126976        0             0 syz-executor3
[  366.510367] [  23767]     0 23767    17618     8218   126976        0             0 syz-executor2
[  366.519409] Memory cgroup out of memory: Kill process 23766 (syz-executor3) score 8252000 or sacrifice child
[  366.529422] Killed process 23766 (syz-executor3) total-vm:70480kB, anon-rss:116kB, file-rss:32768kB, shmem-rss:0kB
[  366.540456] oom_reaper: reaped process 23766 (syz-executor3), now anon-rss:0kB, file-rss:32000kB, shmem-rss:0kB
[  366.550949] syz-executor3 invoked oom-killer: gfp_mask=0x6000c0(GFP_KERNEL), order=0, oom_score_adj=0
[  366.560374] CPU: 1 PID: 23766 Comm: syz-executor3 Not tainted 4.18.0-rc6-next-20180725+ #18
(...snipped...)
[  367.138136] Tasks state (memory values in pages):
[  367.142986] [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
[  367.151889] [  23766]     0 23766    17620     8002   126976        0             0 syz-executor3
[  367.160946] [  23767]     0 23767    17618     8218   126976        0             0 syz-executor2
[  367.169994] Memory cgroup out of memory: Kill process 23767 (syz-executor2) score 8249000 or sacrifice child
[  367.180119] Killed process 23767 (syz-executor2) total-vm:70472kB, anon-rss:104kB, file-rss:32768kB, shmem-rss:0kB
[  367.192101] oom_reaper: reaped process 23767 (syz-executor2), now anon-rss:0kB, file-rss:32000kB, shmem-rss:0kB
[  367.202986] ------------[ cut here ]------------
[  367.207845] Memory cgroup charge failed because of no reclaimable memory! This looks like a misconfiguration or a kernel bug.
[  367.207965] WARNING: CPU: 1 PID: 23767 at mm/memcontrol.c:1710 try_charge+0x734/0x1680
[  367.227540] Kernel panic - not syncing: panic_on_warn set ...