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From: Eric Blake <eblake@redhat.com>
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Date: Thu, 22 Feb 2018 08:21:10 -0600
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Subject: Re: [Qemu-devel] [PATCH v2 3/3] qcow2: Avoid memory over-allocation
 on compressed images
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To: Alberto Garcia <berto@igalia.com>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>

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