Re: [PATCH v2 1/2] migration: Fix rdma migration failed

[Juan Quintela]

Li Zhijian <lizhijian@fujitsu.com> wrote:
> Migration over RDMA failed since
> commit: 294e5a4034 ("multifd: Only flush once each full round of memory")
> with erors:
> qemu-system-x86_64: rdma: Too many requests in this message 
> (3638950032).Bailing.
>
> migration 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(
> RAM_SAVE_FLAG_MULTIFD_FLUSH) to destination and cause migration to fail
> even though multifd is disabled.
>
> This change make migrate_multifd_flush_after_each_section() return true
> when multifd is disabled, that also means RAM_SAVE_FLAG_MULTIFD_FLUSH
> will not be sent to destination any more when multifd is disabled.
>
> Fixes: 294e5a4034 ("multifd: Only flush once each full round of memory")
> CC: Fabiano Rosas <farosas@suse.de>
> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>

Ouch.

> index 1d1e1321b0..327bcf2fbe 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)

But I think this is ugly.

migrate_multifd_flush_after_each_section()

returnls true

with multifd not enabled?

And we are creating a "function" that just reads a property now does
something else.

What about this?

I know that the change is bigger, but it makes clear what is happening
here.

commit c638f66121ce30063fbf68c3eab4d7429cf2b209
Author: Juan Quintela <quintela@redhat.com>
Date:   Tue Oct 3 20:53:38 2023 +0200

    migration: Non multifd migration don't care about multifd flushes
    
    RDMA was having trouble because
    migrate_multifd_flush_after_each_section() can only be true or false,
    but we don't want to send any flush when we are not in multifd
    migration.
    
    CC: Fabiano Rosas <farosas@suse.de
    Reported-by: Li Zhijian <lizhijian@fujitsu.com>
    Signed-off-by: Juan Quintela <quintela@redhat.com>

diff --git a/migration/ram.c b/migration/ram.c
index e4bfd39f08..716cef6425 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1387,7 +1387,8 @@ 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) {
@@ -3064,7 +3065,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
         return ret;
     }
 
-    if (!migrate_multifd_flush_after_each_section()) {
+    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
         qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
     }
 
@@ -3176,7 +3177,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
 out:
     if (ret >= 0
         && migration_is_setup_or_active(migrate_get_current()->state)) {
-        if (migrate_multifd_flush_after_each_section()) {
+        if (migrate_multifd() && migrate_multifd_flush_after_each_section()) {
             ret = 
multifd_send_sync_main(rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
             if (ret < 0) {
                 return ret;
@@ -3253,7 +3254,7 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
         return ret;
     }
 
-    if (!migrate_multifd_flush_after_each_section()) {
+    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
         qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
     }
     qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
@@ -3760,7 +3761,7 @@ int ram_load_postcopy(QEMUFile *f, int channel)
             break;
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
-            if (migrate_multifd_flush_after_each_section()) {
+            if (migrate_multifd() && 
migrate_multifd_flush_after_each_section()) {
                 multifd_recv_sync_main();
             }
             break;
@@ -4038,7 +4039,8 @@ static int ram_load_precopy(QEMUFile *f)
             break;
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
-            if (migrate_multifd_flush_after_each_section()) {
+            if (migrate_multifd() &&
+                migrate_multifd_flush_after_each_section()) {
                 multifd_recv_sync_main();
             }
             break;