Re: [Qemu-devel] [PATCH 1/2] qcow2: Give the refcount cache the minimum possible size by default

[Eric Blake]

On 03/13/2018 01:48 PM, Alberto Garcia wrote:
> On Tue 13 Mar 2018 07:23:36 PM CET, Eric Blake wrote:
> > > +                *refcount_cache_size =
> > > +                    MIN(combined_cache_size, min_refcount_cache);
> >
> > but here, if combined_cache_size is smaller than min_refcount_cache,
> >
> > > +                *l2_cache_size = combined_cache_size - *refcount_cache_size;
> >
> > then l2_cache_size is set to a negative value.
>
> No, it's set to 0.
>
> If combined == 4k and min_refcount == 256, then
>
>     refcount_cache_size = MIN(4k, 256k) // 4k
>     l2_cache_size = 4k - 4k; // 0

Ah. Mental breakdown on my part in trying to compute (x - MIN()).

> Then the caller ensures that it's always set to the minimum (as it did
> with the previous code).

So the caller will use larger than the requested limits if the requested 
limits are too small, and we are okay with calculations resulting in 0 
here.  All right, thanks for stepping me through my error; you're good 
to go with:

Reviewed-by: Eric Blake <address@hidden>

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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