Re: [Qemu-devel] [PATCH] util/hbitmaps: recalculate count on merge

[Eric Blake]

On 9/27/18 2:31 AM, Vladimir Sementsov-Ogievskiy wrote:
> 27.09.2018 00:28, John Snow wrote:
> > We have been neglecting to do so, which results in wrong counts
> > after merge. In the worst case, we may think the bitmap is empty
> > when it has had new writes merged into it.
> >
> > Reported-by: Eric Blake <address@hidden>
> > Signed-off-by: John Snow <address@hidden>
> > ---
> >   util/hbitmap.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/util/hbitmap.c b/util/hbitmap.c
> > index d5aca5159f..28e9c523ab 100644
> > --- a/util/hbitmap.c
> > +++ b/util/hbitmap.c
> > @@ -759,6 +759,9 @@ bool hbitmap_merge(const HBitmap *a, const HBitmap 
> > *b, HBitmap *result)
> >           }
> >       }
> > +    /* Recompute the dirty count */
> > +    a->count = hb_count_between(a, 0, a->size - 1);
>
> hmm, just looking at function header: shouldn't we update result->count 
> instead? This code shouldn't compile (thanks to my const*)

Hmm. It looks like John and I posted essentially the same patch, but 
that John's version is based against his out-of-tree patch queue, which 
includes Vladimir's "dirty-bitmap: make it possible to restore bitmap 
after merge".

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org