Re: [Qemu-devel] RFC: exit on incoming exec migrate failure

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Dec 09, 2009 at 01:10:18PM -0800, Andrew Farmer wrote:
> Right now, if an incoming migrate through exec fails, the qemu process 
> will end up chewing CPU indefinitely - it looks like it closes the 
> migration FD but doesn't remove its IO handler properly. An easy way 
> to reproduce this is to try launching with -incoming exec:/bin/false.
> This is obviously useless, but illustrates the issue handily.

I've hit this in real life too, with restore from a file containing
the saved state which had got corrupted/truncated. I only discovered
the failure when I wondered by QEMU was chewing 100% cpu

> One solution might be to retry the command on migrate failure, but that
> won't really help in all circumstances (for instance, if the migration 
> command is broken!), so it seems equally appropriate to just die if an 
> incoming exec migration fails. The patch is trivial, and follows - does
> this look sensible? (I'm new to qemu development, but trying to pick it up.)

It looks like a reasonable approach to me. If we carried on running, it would
be hard for apps to determine whether migration succeeded & thus QEMU is
running, or whether it failed and is just idling. By exiting we give the
management app/user the optional to retry simply by relaunching

> diff --git a/migration-exec.c b/migration-exec.c
> index c830669..0292c19 100644
> --- a/migration-exec.c
> +++ b/migration-exec.c
> @@ -114,7 +114,7 @@ static void exec_accept_incoming_migration(void *opaque)
>      ret = qemu_loadvm_state(f);
>      if (ret < 0) {
>          fprintf(stderr, "load of migration failed\n");
> -        goto err;
> +        exit(0);
>      }
>      qemu_announce_self();
>      dprintf("successfully loaded vm state\n");
> @@ -123,7 +123,6 @@ static void exec_accept_incoming_migration(void *opaque)
>      if (autostart)
>          vm_start();
>  
> -err:
>      qemu_fclose(f);
>  }


Daniel
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