Re: [PATCH 1/2] migration: Fix rdma migration failed

[Peter Xu]

On Wed, Sep 20, 2023 at 05:04:11PM +0800, Li Zhijian wrote:
> From: Li Zhijian <lizhijian@cn.fujitsu.com>
> 
> Destination will fail with:
> qemu-system-x86_64: rdma: Too many requests in this message 
> (3638950032).Bailing.
> 
> migrate with RDMA is different from tcp. RDMA has its own control
> message, and all traffic between RDMA_CONTROL_REGISTER_REQUEST and
> RDMA_CONTROL_REGISTER_FINISHED should not be disturbed.
> 
> find_dirty_block() will be called during RDMA_CONTROL_REGISTER_REQUEST
> and RDMA_CONTROL_REGISTER_FINISHED, it will send a extra traffic to
> destination and cause migration to fail.
> 
> Since there's no existing subroutine to indicate whether it's migrated
> by RDMA or not, and RDMA is not compatible with multifd, we use
> migrate_multifd() here.
> 
> Fixes: 294e5a4034 ("multifd: Only flush once each full round of memory")
> Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
> ---
>  migration/ram.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 9040d66e61..89ae28e21a 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1399,7 +1399,8 @@ static int find_dirty_block(RAMState *rs, 
> PageSearchStatus *pss)
>          pss->page = 0;
>          pss->block = QLIST_NEXT_RCU(pss->block, next);
>          if (!pss->block) {
> -            if (!migrate_multifd_flush_after_each_section()) {
> +            if (migrate_multifd() &&
> +                !migrate_multifd_flush_after_each_section()) {
>                  QEMUFile *f = rs->pss[RAM_CHANNEL_PRECOPY].pss_channel;
>                  int ret = multifd_send_sync_main(f);
>                  if (ret < 0) {
> -- 
> 2.31.1
> 

Maybe better to put that check at the entry of
migrate_multifd_flush_after_each_section()?

I also hope that some day there's no multifd function called in generic
migration code paths..

-- 
Peter Xu