Re: [PATCH 2/2] dump-guest-memory: Block live migration

[Peter Xu <peterx@redhat.com>]

On Tue, Aug 24, 2021 at 10:04:19PM +0400, Marc-André Lureau wrote:
> Hi

Hello, Marc-Andre,

> 
> On Tue, Aug 24, 2021 at 7:27 PM Peter Xu <peterx@redhat.com> wrote:
> 
> > Both dump-guest-memory and live migration caches vm state at the beginning.
> > Either of them entering the other one will cause race on the vm state, and
> > even
> > more severe on that (please refer to the crash report in the bug link).
> >
> > Let's block live migration in dump-guest-memory, and that'll also block
> > dump-guest-memory if it detected that we're during a live migration.
> >
> 
> How does it detect that migration is in progress?

migrate_add_blocker() (and the new migrate_add_blocker_internal()) guaranteed
it; it will only succeed if there's no migration, and it should cover both
sides of migration (as I think migration_is_idle() should return true on both
src/dst when there's one):

    if (migration_is_idle()) {
        migration_blockers = g_slist_prepend(migration_blockers, reason);
        return 0;
    }

    error_propagate_prepend(errp, error_copy(reason),
                            "disallowing migration blocker "
                            "(migration in progress) for: ");
    return -EBUSY;

That's why I removed the old check on incoming migration:

-    if (runstate_check(RUN_STATE_INMIGRATE)) {
-        error_setg(errp, "Dump not allowed during incoming migration.");
-        return;
-    }

Because I think it'll cover that case too.

Thanks,

-- 
Peter Xu