Re: [Qemu-devel] [Qemu-block] [PATCH for-2.6 2/2] block/mirror: Refresh stale bitmap iterator cache

[Kevin Wolf <kwolf@redhat.com>]

Am 20.04.2016 um 14:13 hat Kevin Wolf geschrieben:
> Am 20.04.2016 um 00:59 hat Max Reitz geschrieben:
> > If the drive's dirty bitmap is dirtied while the mirror operation is
> > running, the cache of the iterator used by the mirror code may become
> > stale and not contain all dirty bits.
> > 
> > This only becomes an issue if we are looking for contiguously dirty
> > chunks on the drive. In that case, we can easily detect the discrepancy
> > and just refresh the iterator if one occurs.
> > 
> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > ---
> >  block/mirror.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/block/mirror.c b/block/mirror.c
> > index 2714a77..9df1fae 100644
> > --- a/block/mirror.c
> > +++ b/block/mirror.c
> > @@ -334,6 +334,11 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
> >          }
> >  
> >          hbitmap_next = hbitmap_iter_next(&s->hbi);
> > +        if (hbitmap_next > next_sector || hbitmap_next < 0) {
> > +            /* The bitmap iterator's cache is stale, refresh it */
> > +            bdrv_set_dirty_iter(&s->hbi, next_sector);
> > +            hbitmap_next = hbitmap_iter_next(&s->hbi);
> > +        }
> >          assert(hbitmap_next == next_sector);
> 
> The iterator doesn't seem to be used afterwards anyway, so why not just
> use next_sector and stop using the iterator when we already know what
> result we want to get?

And of course I read that code completely wrong because in reality
&s->hbi isn't local. Seems to be the right way to do things then.

Kevin