Re: [PATCH v3] migration: Don't allow migration if vm is in POSTMIGRATE

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Tuguoyi (tu.guoyi@h3c.com) wrote:
> The following steps will cause qemu assertion failure:
> - pause vm by executing 'virsh suspend'
> - create external snapshot of memory and disk using 'virsh snapshot-create-as'
> - doing the above operation again will cause qemu crash
> 
> The backtrace looks like:
> #0  0x00007fbf958c5c37 in raise () from /lib/x86_64-linux-gnu/libc.so.6
> #1  0x00007fbf958c9028 in abort () from /lib/x86_64-linux-gnu/libc.so.6
> #2  0x00007fbf958bebf6 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> #3  0x00007fbf958beca2 in __assert_fail () from /lib/x86_64-linux-gnu/libc.so.6
> #4  0x000055ca8decd39d in bdrv_inactivate_recurse (bs=0x55ca90c80400) at /build/qemu-5.0/block.c:5724
> #5  0x000055ca8dece967 in bdrv_inactivate_all () at /build//qemu-5.0/block.c:5792
> #6  0x000055ca8de5539d in qemu_savevm_state_complete_precopy_non_iterable (inactivate_disks=true, in_postcopy=false, f=0x55ca907044b0)
>     at /build/qemu-5.0/migration/savevm.c:1401
> #7  qemu_savevm_state_complete_precopy (f=0x55ca907044b0, iterable_only=iterable_only@entry=false, inactivate_disks=inactivate_disks@entry=true)
>     at /build/qemu-5.0/migration/savevm.c:1453
> #8  0x000055ca8de4f581 in migration_completion (s=0x55ca8f64d9f0) at /build/qemu-5.0/migration/migration.c:2941
> #9  migration_iteration_run (s=0x55ca8f64d9f0) at /build/qemu-5.0/migration/migration.c:3295
> #10 migration_thread (opaque=opaque@entry=0x55ca8f64d9f0) at /build/qemu-5.0/migration/migration.c:3459
> #11 0x000055ca8dfc6716 in qemu_thread_start (args=<optimized out>) at /build/qemu-5.0/util/qemu-thread-posix.c:519
> #12 0x00007fbf95c5f184 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
> #13 0x00007fbf9598cbed in clone () from /lib/x86_64-linux-gnu/libc.so.6
> 
> When the first migration completes, bs->open_flags will set BDRV_O_INACTIVE
> flag by bdrv_inactivate_all(), and during the second migration the
> bdrv_inactivate_recurse assert that the bs->open_flags is already
> BDRV_O_INACTIVE enabled which cause crash.
> 
> As Vladimir suggested, this patch makes migrate_prepare check the state of vm and
> return error if it is in RUN_STATE_POSTMIGRATE state.
> 
> Signed-off-by: Tuguoyi <tu.guoyi@h3c.com>

Yes, we've had this problem for a long long time; there are a bunch of
other similar conditions; the real answer is to figure out some command
for explicitly handing back control of the block devices (without
actually restarting the CPUs).

However, this is a reasonable patch to cover the common case.

Queued

> ---
>  migration/migration.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 87a9b59..5e33962 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2115,6 +2115,12 @@ static bool migrate_prepare(MigrationState *s, bool blk, bool blk_inc,
>          return false;
>      }
>  
> +    if (runstate_check(RUN_STATE_POSTMIGRATE)) {
> +        error_setg(errp, "Can't migrate the vm that was paused due to "
> +                   "previous migration");
> +        return false;
> +    }
> +
>      if (migration_is_blocked(errp)) {
>          return false;
>      }
> -- 
> 2.7.4
> 
> [Patch v2]: https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg01318.html
> [Patch v1]: https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg05950.html
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK