Re: [Qemu-devel] [PULL 20/37] qcow2: Give the refcount cache the minimum possible size by default

[Kevin Wolf <kwolf@redhat.com>]

Am 25.05.2018 um 19:10 hat Peter Maydell geschrieben:
> On 15 May 2018 at 16:40, Kevin Wolf <kwolf@redhat.com> wrote:
> > From: Alberto Garcia <berto@igalia.com>
> >
> > The L2 and refcount caches have default sizes that can be overridden
> > using the l2-cache-size and refcount-cache-size (an additional
> > parameter named cache-size sets the combined size of both caches).
> 
> Hi; Coverity raises a nit about this patch (CID 1391229):

CCing Berto as the patch author.

> > diff --git a/block/qcow2.c b/block/qcow2.c
> > index 2f36e632f9..6d532470a8 100644
> > --- a/block/qcow2.c
> > +++ b/block/qcow2.c
> > @@ -802,23 +802,30 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
> >          } else if (refcount_cache_size_set) {
> >              *l2_cache_size = combined_cache_size - *refcount_cache_size;
> >          } else {
> > -            *refcount_cache_size = combined_cache_size
> > -                                 / (DEFAULT_L2_REFCOUNT_SIZE_RATIO + 1);
> > -            *l2_cache_size = combined_cache_size - *refcount_cache_size;
> > +            uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
> > +            uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
> > +            uint64_t min_refcount_cache =
> > +                (uint64_t) MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
> 
> Here we have a (uint64_t) cast that ensures we're doing a 64x64 multiply
> rather than a 32x32 one...
> 
> > +
> > +            /* Assign as much memory as possible to the L2 cache, and
> > +             * use the remainder for the refcount cache */
> > +            if (combined_cache_size >= max_l2_cache + min_refcount_cache) {
> > +                *l2_cache_size = max_l2_cache;
> > +                *refcount_cache_size = combined_cache_size - *l2_cache_size;
> > +            } else {
> > +                *refcount_cache_size =
> > +                    MIN(combined_cache_size, min_refcount_cache);
> > +                *l2_cache_size = combined_cache_size - *refcount_cache_size;
> > +            }
> >          }
> >      } else {
> > -        if (!l2_cache_size_set && !refcount_cache_size_set) {
> > +        if (!l2_cache_size_set) {
> >              *l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
> >                                   (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
> >                                   * s->cluster_size);
> > -            *refcount_cache_size = *l2_cache_size
> > -                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
> > -        } else if (!l2_cache_size_set) {
> > -            *l2_cache_size = *refcount_cache_size
> > -                           * DEFAULT_L2_REFCOUNT_SIZE_RATIO;
> > -        } else if (!refcount_cache_size_set) {
> > -            *refcount_cache_size = *l2_cache_size
> > -                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
> > +        }
> > +        if (!refcount_cache_size_set) {
> > +            *refcount_cache_size = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
> 
> ...but in the else clause down here, we don't have the cast, and
> Coverity complains that we evaluate a 32-bit result and then
> put it in a 64-bit variable. Should this have the (uint64_t)
> cast as well ?

It's a false positive, MIN_REFCOUNT_CACHE_SIZE is 4 and s->cluster_size
is at most 2 MB, so this will never overflow.

I guess we can change the code anyway to silence it?

Kevin