Re: qcow2 merge_cow() question

[Vladimir Sementsov-Ogievskiy]

21.08.2020 15:52, Alberto Garcia wrote:
> On Fri 21 Aug 2020 02:32:00 PM CEST, Vladimir Sementsov-Ogievskiy wrote:
> > Hi!
> >
> > I'm sorry, I feel I already asked about it, but can't now find the answer.
> >
> > What are these ifs for?
> >
> >             /* The data (middle) region must be immediately after the
> >              * start region */
> >             if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
> >                 continue;
> >             }
> >                                                                                                             
> >             /* The end region must be immediately after the data (middle)
> >              * region */
> >             if (m->offset + m->cow_end.offset != offset + bytes) {
> >                 continue;
> >             }
> >
> > How is it possible that data doesn't immediately follow start cow region or
> > end cow region doesn't immediately follow data region?
>
> They are sanity checks. They maybe cannot happen in practice and in that
> case I suppose they should be replaced with assertions but this should
> be checked carefully. If I remember correctly I was wary of overlooking
> a case where this could happen.
>
> In particular, that function receives only one data region but a list of
> QCowL2Meta objects. I think you can get more than one QCowL2Meta if the
> same request involves a mix of copied and newly allocated clusters, but
> that shouldn't be a problem either.
>
> Berto

OK, thanks. So, intuitively it shouldn't happen, but there should be
some careful investigation to change them to assertions.

I think that the list actually may contain not more than 2 entries.
So the whole diversity is that we may have both cow regions in one
meta, or in two separate metas.

--
Best regards,
Vladimir