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

[Zhijian Li (Fujitsu)]

On 22/09/2023 23:42, Peter Xu wrote:
> 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()?
> 

It sounds good to me:
diff --git a/migration/options.c b/migration/options.c
index 1d1e1321b0a..327bcf2fbe4 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -368,7 +368,7 @@ bool migrate_multifd_flush_after_each_section(void)
  {
      MigrationState *s = migrate_get_current();

-    return s->multifd_flush_after_each_section;
+    return !migrate_multifd() || s->multifd_flush_after_each_section;
  }

  bool migrate_postcopy(void)


That changes make migrate_multifd_flush_after_each_section() always true when 
multifd is disabled.

Thanks



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