Re: [PATCH RFC 01/12] migration: Fix low possibility downtime violation

[Juraj Marcin <jmarcin@redhat.com>]

On 2026-03-19 19:12, Peter Xu wrote:
> When QEMU queried the estimated version of pending data and thinks it's
> ready to converge, it'll send another accurate query to make sure of it.
> It is needed to make sure we collect the latest reports and that equation
> still holds true.
> 
> However we missed one tiny little difference here on "<" v.s. "<=" when
> comparing pending_size (A) to threshold_size (B)..
> 
> QEMU src only re-query if A<B, but will kickoff switchover if A<=B.
> 
> I think it means it is possible to happen if A (as an estimate only so far)
> accidentally equals to B, then re-query won't happen and switchover will
> proceed without considering new dirtied data.
> 
> It turns out it was an accident in my commit 7aaa1fc072 when refactoring
> the code around.  Fix this by using the same equation in both places.
> 
> Fixes: 7aaa1fc072 ("migration: Rewrite the migration complete detect logic")
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 5c9aaa6e58..dfc60372cf 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -3242,7 +3242,7 @@ static MigIterateState migration_iteration_run(MigrationState *s)
>           * postcopy started, so ESTIMATE should always match with EXACT
>           * during postcopy phase.
>           */
> -        if (pending_size < s->threshold_size) {
> +        if (pending_size <= s->threshold_size) {
>              qemu_savevm_state_pending_exact(&must_precopy, &can_postcopy);
>              pending_size = must_precopy + can_postcopy;
>              trace_migrate_pending_exact(pending_size, must_precopy,
> -- 
> 2.50.1
> 

Reviewed-by: Juraj Marcin <jmarcin@redhat.com>