Re: [Qemu-devel] [PATCH v6 4/9] qcow2: make refcount size calculation conservative

[Max Reitz <mreitz@redhat.com>]

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On 08.05.2017 17:00, Eric Blake wrote:
> On 05/08/2017 09:15 AM, Stefan Hajnoczi wrote:
>> The refcount metadata size calculation is inaccurate and can produce
>> numbers that are too small.  This is bad because we should calculate a
>> conservative number - one that is guaranteed to be large enough.
>>
>> This patch switches the approach to a fixed point calculation because
>> the existing equation is hard to solve when inaccuracies are taken care
>> of.
>>
>> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
>> Reviewed-by: Alberto Garcia <berto@igalia.com>
>> ---
>>  block/qcow2.c | 82 ++++++++++++++++++++++++++++++-----------------------------
>>  1 file changed, 42 insertions(+), 40 deletions(-)
>>
> 
>> -    nrefblocke = (aligned_total_size + meta_size + cluster_size)
>> -        / (cluster_size - rces - rces * sizeof(uint64_t)
>> -                / cluster_size);
>> -    meta_size += DIV_ROUND_UP(nrefblocke, refblock_size) * cluster_size;
>> -
>> -    /* total size of refcount tables */
>> -    nreftablee = nrefblocke / refblock_size;
>> -    nreftablee = align_offset(nreftablee, cluster_size / sizeof(uint64_t));
>> -    meta_size += nreftablee * sizeof(uint64_t);
>> +    /* total size of refcount table and blocks */
>> +    meta_size += qcow2_refcount_metadata_size(
>> +            (meta_size + aligned_total_size) / cluster_size,
> 
> How does this interact with Max's patch which avoids truncating division
> in favor of rounding up?
> https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg00690.html

From a quick glance, it looks like it just reimplements everything in a
(hopefully) more understandable and "generous" way (which is good). So
the only interaction is that the code that's removed/replaced is a bit
different (which implies a rebase conflict, but one that should be
resolved pretty easily).

Max


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