Re: [PATCH for-9.0 1/2] migration: Set migration error in migration_completion()

[Cédric Le Goater]

On 3/28/24 16:50, Avihai Horon wrote:

On 28/03/2024 17:21, Cédric Le Goater wrote:

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Hello Avihai,

On 3/28/24 15:02, Avihai Horon wrote:

After commit 9425ef3f990a ("migration: Use migrate_has_error() in
close_return_path_on_source()"), close_return_path_on_source() assumes
that migration error is set if an error occurs during migration.

This may not be true if migration errors in migration_completion(). For
example, if qemu_savevm_state_complete_precopy() errors, migration error
will not be set

Out of curiosity, could you describe a bit more the context ? Did
vfio_save_complete_precopy() fail ? why ?

Yep, vfio_save_complete_precopy() failed (but it failed while I was 
experimenting with an unofficial debug FW).

We should propagate errors of .save_live_complete_precopy() handlers as
it was done .save_setup handlers(). For 9.1.

Agreed.

This in turn, will cause a migration hang bug, similar to the bug that
was fixed by commit 22b04245f0d5 ("migration: Join the return path
thread before releasing to_dst_file"), as shutdown() will not be issued
for the return-path channel.

yes, but this test :

    if (ret < 0) {
        goto fail;
    }

will skip the close_return_path_on_source() call. Won't it ? So I don't
understand how it can be an issue. Am I missing something ?

It will skip the close_return_path_on_source() call in migration_completion(), 
but there is another close_return_path_on_source() call in migrate_fd_cleanup().

OK. Found it. This is a code path I hadn't explored yet.

Acked-by: Cédric Le Goater <c...@redhat.com>

Thanks,

C.

Fix it by ensuring migration error is set in case of error in
migration_completion().

Why didn't you add a reference to commit 9425ef3f990a ?

I thought this commit didn't introduce this bug, but looking again in the 
mailing list [1], it kinda did:
The hang bug was fully fixed by commit 22b04245f0d ("migration: Join the return path 
thread before releasing to_dst_file") and then 9425ef3f990a re-introduced the bug, 
but only for migration_completion() case.
So, you are right, a fixes line with 9425ef3f990a should be added.

Thanks.

[1] https://lore.kernel.org/all/20240226203122.22894-1-faro...@suse.de/

Signed-off-by: Avihai Horon <avih...@nvidia.com>
---
  migration/migration.c | 10 ++++++++++
  1 file changed, 10 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index 9fe8fd2afd7..b73ae3a72c4 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2799,6 +2799,7 @@ static void migration_completion(MigrationState *s)
  {
      int ret = 0;
      int current_active_state = s->state;
+    Error *local_err = NULL;

      if (s->state == MIGRATION_STATUS_ACTIVE) {
          ret = migration_completion_precopy(s, &current_active_state);
@@ -2832,6 +2833,15 @@ static void migration_completion(MigrationState *s)
      return;

  fail:
+    if (qemu_file_get_error_obj(s->to_dst_file, &local_err)) {
+        migrate_set_error(s, local_err);
+        error_free(local_err);
+    } else if (ret) {
+        error_setg_errno(&local_err, -ret, "Error in migration completion");

The 'ret = -1' case could be improved with error_setg(). As a followup.

Thanks,

C.

+        migrate_set_error(s, local_err);
+        error_free(local_err);
+    }
+
      migration_completion_failed(s, current_active_state);
  }