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

[Peter Xu]

On Fri, Sep 22, 2023 at 12:59:37PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > 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..
> 
> I wonder what happened with that MigrationOps idea. We added the
> ram_save_target_page pointer and nothing else. It seems like it could be
> something in the direction of allowing different parts of the migration
> code to provide different behavior without having to put these explicit
> checks all over the place.

Yeah..

https://lore.kernel.org/qemu-devel/20230130080956.3047-12-quintela@redhat.com/

Juan should know better.

Personally I think it'll be good we only introduce hook when there's a 2nd
user already.  I assume Juan merged it planning that'll land soon but it
didn't.

-- 
Peter Xu