Re: [PATCH v2 3/7] migration: Fix socket listen backlog

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> QEMU incoming side may have set a wrong listen() backlog.  After the
> support of multifd + postcopy now we have four possibilities on setting the
> backlog, some of them are wrong:
>
>   Vanilla:            1 (correct)
>   Multifd:            N (wrong, should be N+1)
>   Preempt:            2 (correct)
>   Multifd+Preempt:    N (wrong, should be N+2)
>
> No real report for this, found by code observation only.  Logically the
> backlog (pending accept requests on destination on the listening sockets)
> needs to be the maximum of possible channels.  Otherwise if the accept
> queue full on destination, source QEMU may hit connection errors.
>
> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Fixes: 0705e56496 ("multifd: Use number of channels as listen backlog")
> Fixes: e274188612 ("migration: enable multifd and postcopy together")
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/socket.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/migration/socket.c b/migration/socket.c
> index f57f1a8e7c..31bfa1eeeb 100644
> --- a/migration/socket.c
> +++ b/migration/socket.c
> @@ -139,21 +139,32 @@ socket_incoming_migration_end(void *opaque)
>      object_unref(OBJECT(listener));
>  }
>  
> +static int socket_get_max_channels(void)
> +{
> +    /* The main channel is always wanted */
> +    int num = 1;
> +
> +    if (migrate_multifd()) {
> +        num += migrate_multifd_channels();
> +    }
> +
> +    if (migrate_postcopy_preempt()) {
> +        /* The preempt channel */
> +        num += 1;
> +    }
> +
> +    return num;
> +}
> +
>  void socket_connect_incoming(SocketAddress *saddr, Error **errp)
>  {
>      QIONetListener *listener = qio_net_listener_new();
>      MigrationIncomingState *mis = migration_incoming_get_current();
>      size_t i;
> -    int num = 1;
> +    int num = socket_get_max_channels();
>  
>      qio_net_listener_set_name(listener, "migration-socket-listener");
>  
> -    if (migrate_multifd()) {
> -        num = migrate_multifd_channels();
> -    } else if (migrate_postcopy_preempt()) {
> -        num = RAM_CHANNEL_MAX;
> -    }
> -
>      if (qio_net_listener_open_sync(listener, saddr, num, errp) < 0) {
>          object_unref(OBJECT(listener));
>          return;

Reviewed-by: Fabiano Rosas <farosas@suse.de>