Re: INFO: task hung in ext4_da_get_block_prep

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Sep 5, 2018 at 12:53 PM, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
> On 2018/09/05 16:22, Dmitry Vyukov wrote:
>> On Wed, Sep 5, 2018 at 5:41 AM, syzbot
>> <syzbot+f0fc7f62e88b1de99af3@syzkaller.appspotmail.com> wrote:
>>> Hello,
>>>
>>> syzbot found the following crash on:
>>>
>>> HEAD commit:    f2b6e66e9885 Add linux-next specific files for 20180904
>>> git tree:       linux-next
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1735dc92400000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=15ad48400e39c1b3
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=f0fc7f62e88b1de99af3
>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>> CC:             [adilger.kernel@dilger.ca linux-ext4@vger.kernel.org
>>> linux-kernel@vger.kernel.org tytso@mit.edu]
>>>
>>> Unfortunately, I don't have any reproducer for this crash yet.
>>>
>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>> Reported-by: syzbot+f0fc7f62e88b1de99af3@syzkaller.appspotmail.com
>>>
>>> [   7961]     0  7961    17585     8737   131072        0             0
>>> syz-executor3
>>
>> Hi Tetsuo,
>>
>> Maybe you know what are these repeated lines with numbers?
>> We started getting them on linux-next recently, also:
>> https://syzkaller.appspot.com/bug?extid=f8fa79b458bcae4d913d
>> They seem to cause various hangs/stalls.
>
> Yes, these lines are from the OOM killer. (Thus, if we can, I want to
> remove ext4 people before upstreaming this report.)

This is not possible at the moment.

>   dump_tasks mm/oom_kill.c:420 [inline]
>   dump_header+0xf0d/0xf70 mm/oom_kill.c:450
>   oom_kill_process.cold.28+0x10/0x95a mm/oom_kill.c:953
>   out_of_memory+0xa88/0x1430 mm/oom_kill.c:1120
>
> What is annoying is that one for_each_process() traversal with printk() is
> taking 52 seconds which is too long to do under RCU section. Under such
> situation, invoking the OOM killer for three times will exceed khungtaskd
> threshold 140 seconds. Was syzbot trying to test fork bomb situation?

Hard to tell. I only know what's captured in the console output.

> Anyway, we might need to introduce rcu_lock_break() like
> check_hung_uninterruptible_tasks() does...
>
> [  999.629589] [  16497]     0 16497    17585     8739   126976        0             0 syz-executor5
> [ 1026.435955] [  32764]     0 32764    17585     8739   126976        0             0 syz-executor5
> [ 1026.445027] [    311]     0   311    17585     8737   131072        0             0 syz-executor3
> [ 1047.914324] [  10315]     0 10315    17585     8271   126976        0             0 syz-executor0
> [ 1047.923384] Out of memory: Kill process 4670 (syz-fuzzer) score 53 or sacrifice child
> [ 1047.931934] Killed process 5032 (syz-executor1) total-vm:70212kB, anon-rss:60kB, file-rss:0kB, shmem-rss:0kB
> [ 1047.988138] syz-executor2 invoked oom-killer: gfp_mask=0x6040c0(GFP_KERNEL|__GFP_COMP), nodemask=(null), order=1, oom_score_adj=0
> [ 1048.000015] syz-executor2 cpuset=syz2 mems_allowed=0
> [ 1048.005199] CPU: 0 PID: 4700 Comm: syz-executor2 Not tainted 4.19.0-rc2-next-20180904+ #55
> [ 1048.740679] [   2347]     0  2347      278      186    32768        0             0 none
> [ 1051.319928] [  16497]     0 16497    17585     8739   126976        0             0 syz-executor5
> [ 1096.740878] [   8841]     0  8841    17585     8232   126976        0             0 syz-executor5
> [ 1078.140677] [  32764]     0 32764    17585     8739   126976        0             0 syz-executor5
> [ 1078.149807] [    311]     0   311    17585     8737   131072        0             0 syz-executor3
> [ 1096.740878] [   8841]     0  8841    17585     8232   126976        0             0 syz-executor5
>
> Also, another notable thing is that the backtrace for some reason includes
>
> [ 1048.211540]  ? oom_killer_disable+0x3a0/0x3a0
>
> line. Was syzbot testing process freezing functionality?

What's the API for this?