Re: [Qemu-devel] [PATCH v2 07/21] qcow2: Helper function for refcount modification

[Max Reitz <mreitz@redhat.com>]

On 2014-11-15 at 18:02, Eric Blake wrote:
> On 11/14/2014 06:06 AM, Max Reitz wrote:
>> Since refcounts do not always have to be a uint16_t, all refcount blocks
>> and arrays in memory should not have a specific type (thus they become
>> pointers to void) and for accessing them, two helper functions are used
>> (a getter and a setter). Those functions are called indirectly through
>> function pointers in the BDRVQcowState so they may later be exchanged
>> for different refcount orders.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/qcow2-refcount.c | 128 ++++++++++++++++++++++++++++++-------------------
>>   block/qcow2.h          |   8 ++++
>>   2 files changed, 87 insertions(+), 49 deletions(-)
>>
>> @@ -1216,7 +1249,7 @@ enum {
>>    * error occurred.
>>    */
>>   static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
>> -    uint16_t **refcount_table, int64_t *refcount_table_size, int64_t l2_offset,
>> +    void **refcount_table, int64_t *refcount_table_size, int64_t l2_offset,
>>       int flags)
>>   {
> Might be worth fixing the indentation here in addition to all the other
> places you adjusted.  But that's minor.
>
>> @@ -1933,17 +1967,13 @@ write_refblocks:
>>               goto fail;
>>           }
>>   
>> -        on_disk_refblock = qemu_blockalign0(bs->file, s->cluster_size);
>> -        for (i = 0; i < s->refcount_block_size &&
>> -                    refblock_start + i < *nb_clusters; i++)
>> -        {
>> -            on_disk_refblock[i] =
>> -                cpu_to_be16((*refcount_table)[refblock_start + i]);
>> -        }
>> +        /* The size of *refcount_table is always cluster-aligned, therefore the
>> +         * write operation will not overflow */
>> +        on_disk_refblock = (void *)((uintptr_t)*refcount_table +
>> +                                    (refblock_index << s->refcount_block_bits));
> Here is where you are relying on the guarantee that you added in 6/21,
> which is why I ask for that one to mention it.
>
> Nice reduction of a bounce buffer, by the way :)  Worth mentioning in
> the commit message as an intentional part of this commit?

Why not.

>> @@ -2087,7 +2117,7 @@ int qcow2_check_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>>           /* Because the old reftable has been exchanged for a new one the
>>            * references have to be recalculated */
>>           rebuild = false;
>> -        memset(refcount_table, 0, nb_clusters * sizeof(uint16_t));
>> +        memset(refcount_table, 0, nb_clusters * s->refcount_bits / 8);
> Phew; we're safe that this won't overflow; and good that you do the *
> first (if you did the /8 first, it would fail for sub-byte refcounts).

Thanks for catching this, it is wrong (albeit it does the right thing). 
It should use refcount_array_byte_size(), which was in this version of 
the series introduced before this patch, so it's an artifact of swapping 
patch 6 and 7.

Max

> Reviewed-by: Eric Blake <eblake@redhat.com>