[Qemu-devel] safety of migration_bitmap_extend

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

Hi,
  I'm trying to understand why migration_bitmap_extend is correct/safe;
If I understand correctly, you're arguing that:

  1) the migration_bitmap_mutex around the extend, stops any sync's happening
     and so no new bits will be set during the extend.

  2) If migration sends a page and clears a bitmap entry, it doesn't
     matter if we lose the 'clear' because we're copying it as
     we extend it, because losing the clear just means the page
     gets resent, and so the data is OK.

However, doesn't (2) mean that migration_dirty_pages might be wrong?
If a page was sent, the bit cleared, and migration_dirty_pages decremented,
then if we copy over that bitmap and 'set' that bit again then migration_dirty_pages
is too small; that means that either migration would finish too early,
or more likely, migration_dirty_pages would wrap-around -ve and
never finish.

Is there a reason it's really safe?

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK