Re: [Qemu-block] [PATCH v2 3/3] qcow2: Avoid memory over-allocation on compressed images

[Eric Blake]

On 02/22/2018 04:50 AM, Alberto Garcia wrote:
> On Thu 22 Feb 2018 12:39:53 AM CET, Eric Blake wrote:
> > +        assert(!!s->cluster_data == !!s->cluster_cache);
> > +        assert(csize < 2 * s->cluster_size + 512);
> >           if (!s->cluster_data) {
> > -            /* one more sector for decompressed data alignment */
> > -            s->cluster_data = qemu_try_blockalign(bs->file->bs,
> > -                    QCOW_MAX_CRYPT_CLUSTERS * s->cluster_size + 512);
> > +            s->cluster_data = g_try_malloc(2 * s->cluster_size + 512);
> >               if (!s->cluster_data) {
> >                   return -ENOMEM;
> >               }
>
> Why the "+ 512" ?

I was thinking "number of sectors is up to two clusters, and we add one, 
PLUS we must read from the initial sector containing the offset".  But I 
was obviously not careful enough - the maximum value (all 1s) is 512 
bytes short of 2 full clusters, and we add at most 511 more bytes for 
the initial sector containing the offset (our +1 covers the leading 
sector).  So you are right, I can tighten this down for a slightly 
smaller allocation (and a nice power-of-2 allocation may be slightly 
more efficient as well).  v3 coming up.

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