Re: [PATCH v2 5/5] vhost: add device started check in migration set log

[Jason Wang <jasowang@redhat.com>]

On 2020/5/16 上午11:20, Li Feng wrote:
> Hi, Dima.
> This abort is what I have mentioned in my previous email.
> I have triggered this crash without any fix a week ago.
> And I have written a test patch to let vhost_log_global_start return
> int and propagate the error to up layer.
> However, my change is a little large, because the origin callback
> return void, and don't do some rollback.
> After test, the migration could migrate to dst successfully, and fio
> is still running perfectly, but the src vm is still stuck here, no
> crash.
>
> Is it right to return this error to the up layer?


That could be a solution or we may ask David for more suggestion.

Another thing that might be useful is to block re connection during 
migration.

Thanks


>
> Thanks,
> Feng Li
>
> Dima Stepanov <dimastep@yandex-team.ru> 于2020年5月16日周六 上午12:55写道：
>> On Thu, May 14, 2020 at 03:34:24PM +0800, Jason Wang wrote:
>>> On 2020/5/13 下午5:47, Dima Stepanov wrote:
>>>>>>      case CHR_EVENT_CLOSED:
>>>>>>          /* a close event may happen during a read/write, but vhost
>>>>>>           * code assumes the vhost_dev remains setup, so delay the
>>>>>>           * stop & clear to idle.
>>>>>>           * FIXME: better handle failure in vhost code, remove bh
>>>>>>           */
>>>>>>          if (s->watch) {
>>>>>>              AioContext *ctx = qemu_get_current_aio_context();
>>>>>>
>>>>>>              g_source_remove(s->watch);
>>>>>>              s->watch = 0;
>>>>>>              qemu_chr_fe_set_handlers(&s->chr, NULL, NULL, NULL, NULL,
>>>>>>                                       NULL, NULL, false);
>>>>>>
>>>>>>              aio_bh_schedule_oneshot(ctx, chr_closed_bh, opaque);
>>>>>>          }
>>>>>>          break;
>>>>>>
>>>>>> I think it's time we dropped the FIXME and moved the handling to common
>>>>>> code. Jason? Marc-André?
>>>>> I agree. Just to confirm, do you prefer bh or doing changes like what is
>>>>> done in this series? It looks to me bh can have more easier codes.
>>>> Could it be a good idea just to make disconnect in the char device but
>>>> postphone clean up in the vhost-user-blk (or any other vhost-user
>>>> device) itself? So we are moving the postphone logic and decision from
>>>> the char device to vhost-user device. One of the idea i have is as
>>>> follows:
>>>>    - Put ourself in the INITIALIZATION state
>>>>    - Start these vhost-user "handshake" commands
>>>>    - If we got a disconnect error, perform disconnect, but don't clean up
>>>>      device (it will be clean up on the roll back). I can be done by
>>>>      checking the state in vhost_user_..._disconnect routine or smth like it
>>>
>>> Any issue you saw just using the aio bh as Michael posted above.
>>>
>>> Then we don't need to deal with the silent vhost_dev_stop() and we will have
>>> codes that is much more easier to understand.
>> I've implemented this solution inside
>> hw/block/vhost-user-blk.c:vhost_user_blk_event() in the similar way by
>> using the s->connected field. Looks good and more correct fix ). I have
>> two questions here before i'll rework the fixes:
>> 1. Is it okay to make the similar fix inside vhost_user_blk_event() or
>> we are looking for more generic vhost-user solution? What do you think?
>> 2. For migration we require an additional information that for the
>> vhost-user device it isn't an error, because i'm trigerring the
>> following assert error:
>>    Core was generated by `x86_64-softmmu/qemu-system-x86_64 -nodefaults -no-user-config -M q35,sata=false'.
>>    Program terminated with signal SIGABRT, Aborted.
>>    #0  0x00007fb56e729428 in raise () from /lib/x86_64-linux-gnu/libc.so.6
>>    [Current thread is 1 (Thread 0x7fb486ef5700 (LWP 527734))]
>>
>>    (gdb) bt
>>    #0  0x00007fb56e729428 in raise () from /lib/x86_64-linux-gnu/libc.so.6
>>    #1  0x00007fb56e72b02a in abort () from /lib/x86_64-linux-gnu/libc.so.6
>>    #2  0x00005648ea376ee6 in vhost_log_global_start
>>        (listener=0x5648ece4eb08) at ./hw/virtio/vhost.c:857
>>    #3  0x00005648ea2dde7e in memory_global_dirty_log_start ()
>>        at ./memory.c:2611
>>    #4  0x00005648ea2e68e7 in ram_init_bitmaps (rs=0x7fb4740008c0)
>>        at ./migration/ram.c:2305
>>    #5  0x00005648ea2e698b in ram_init_all (rsp=0x5648eb1f0f20 <ram_state>)
>>        at ./migration/ram.c:2323
>>    #6  0x00005648ea2e6cc5 in ram_save_setup (f=0x5648ec609e00,
>>        opaque=0x5648eb1f0f20 <ram_state>)
>>        at ./migration/ram.c:2436
>>    #7  0x00005648ea67b7d3 in qemu_savevm_state_setup (f=0x5648ec609e00) at
>>        migration/savevm.c:1176
>>    #8  0x00005648ea674511 in migration_thread (opaque=0x5648ec031ff0) at
>>        migration/migration.c:3416
>>    #9  0x00005648ea85d65d in qemu_thread_start (args=0x5648ec6057f0) at
>>        util/qemu-thread-posix.c:519
>>    #10 0x00007fb56eac56ba in start_thread () from
>>        /lib/x86_64-linux-gnu/libpthread.so.0
>>    #11 0x00007fb56e7fb41d in clone () from /lib/x86_64-linux-gnu/libc.so.6
>>    (gdb) frame 2
>>    #2  0x00005648ea376ee6 in vhost_log_global_start
>>       (listener=0x5648ece4eb08) at ./hw/virtio/vhost.c:857
>>    857             abort();
>>    (gdb) list
>>    852     {
>>    853         int r;
>>    854
>>    855         r = vhost_migration_log(listener, true);
>>    856         if (r < 0) {
>>    857             abort();
>>    858         }
>>    859     }
>>    860
>>    861     static void vhost_log_global_stop(MemoryListener *listener)
>> Since bh postphone the clean up, we can't use the ->started field.
>> Do we have any mechanism to get the device type/state in the common
>> vhost_migration_log() routine? So for example for the vhost-user/disconnect
>> device we will be able to return 0. Or should we implement it and introduce
>> it in this patch set?
>>
>> Thanks, Dima.
>>
>>> Thank
>>>
>>>
>>>>    - vhost-user command returns error back to the _start() routine
>>>>    - Rollback in one place in the start() routine, by calling this
>>>>      postphoned clean up for the disconnect
>>>>