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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 14, 2018 at 07:42:59PM +0000, Dr. David Alan Gilbert wrote:
> * 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?

Yes. Sorry.

> > 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