Re: [Qemu-devel] [PATCH v4 2/4] migration: API to clear bits of guest free pages from the dirty bitmap

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

* Michael S. Tsirkin (mst@redhat.com) wrote:
> On Wed, Mar 14, 2018 at 06:11:37PM +0000, Dr. David Alan Gilbert wrote:
> > > +            used_len = block->used_length - offset;
> > > +            addr += used_len;
> > > +        }
> > > +
> > > +        start = offset >> TARGET_PAGE_BITS;
> > > +        npages = used_len >> TARGET_PAGE_BITS;
> > > +        ram_state->migration_dirty_pages -=
> > > +                      bitmap_count_one_with_offset(block->bmap, start, npages);
> > > +        bitmap_clear(block->bmap, start, npages);
> > 
> > If this is happening while the migration is running, this isn't safe -
> > the migration code could clear a bit at about the same point this
> > happens, so that the count returned by bitmap_count_one_with_offset
> > wouldn't match the word that was cleared by bitmap_clear.
> > 
> > The only way I can see to fix it is to run over the range using
> > bitmap_test_and_clear_atomic, using the return value to decrement
> > the number of dirty pages.
> > But you also need to be careful with the update of the
> > migration_dirty_pages value itself, because that's also being read
> > by the migration thread.
> > 
> > Dave
> 
> I see that there's migration_bitmap_sync but it does not seem to be

Do you mean bitmap_mutex?

> taken on all paths. E.g. migration_bitmap_clear_dirty and
> migration_bitmap_find_dirty are called without that lock sometimes.
> Thoughts?

Hmm, that doesn't seem to protect much at all!  It looks like it was
originally added to handle hotplug causing the bitmaps to be resized;
that extension code was removed in 66103a5 so that lock can probably go.

I don't see how the lock would help us though; the migration thread is
scanning it most of the time so would have to have the lock held
most of the time.

Dave

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