linux-kernel - Re: INFO: task hung in sync

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Date:   Fri, 9 Feb 2018 18:31:51 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Jan Kara <jack@...e.cz>, Andi Kleen <ak@...ux.intel.com>,
        syzbot <syzbot+283c3c447181741aea28@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>, jlayton@...hat.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Ingo Molnar <mingo@...nel.org>, rgoldwyn@...e.com,
        syzkaller-bugs@...glegroups.com, linux-fsdevel@...r.kernel.org
Subject: Re: INFO: task hung in sync_blockdev



On 02/08/2018 08:17 PM, Dmitry Vyukov wrote:
> On Thu, Feb 8, 2018 at 5:23 PM, Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
>>
>>
>> On 02/08/2018 07:18 PM, Jan Kara wrote:
>>
>>>> By "full kernel crashdump" you mean kdump thing, or something else?
>>>
>>> Yes, the kdump thing (for KVM guest you can grab the memory dump also from
>>> the host in a simplier way and it should be usable with the crash utility
>>> AFAIK).
>>>
>>
>> In QEMU monitor 'dump-guest-memory' command:
>>
>> (qemu) help dump-guest-memory
>> dump-guest-memory [-p] [-d] [-z|-l|-s] filename [begin length] -- dump guest memory into file 'filename'.
>>                         -p: do paging to get guest's memory mapping.
>>                         -d: return immediately (do not wait for completion).
>>                         -z: dump in kdump-compressed format, with zlib compression.
>>                         -l: dump in kdump-compressed format, with lzo compression.
>>                         -s: dump in kdump-compressed format, with snappy compression.
>>                         begin: the starting physical address.
>>                         length: the memory size, in bytes
> 
> 
> Nice!
> Do you know straight away if it's scriptable/automatable? Or do I just
> send some magic sequence of bytes representing ^A+C,
> dump-guest-memory, \n to stdin pipe?
> 

I wouldn't do it via stdin. You can setup monitor on any chardev you like and send command
there when you know that guest paniced. Look for -mon and -chardev qemu options.

> Unfortunately, syzbot uses GCE VMs for testing, and there does not
> seem to be such feature on GCE...
> 

Well, you still have kdump.