Re: [Qemu-devel] [PATCH v3 06/22] qcow2: Helper for refcount array reallocation

[Max Reitz <mreitz@redhat.com>]

On 2014-11-27 at 16:09, Stefan Hajnoczi wrote:
> On Thu, Nov 20, 2014 at 06:06:22PM +0100, Max Reitz wrote:
>> +/**
>> + * Reallocates *array so that it can hold new_size entries. *size must contain
>> + * the current number of entries in *array. If the reallocation fails, *array
>> + * and *size will not be modified and -errno will be returned. If the
>> + * reallocation is successful, *array will be set to the new buffer and *size
>> + * will be set to new_size. The size of the reallocated refcount array buffer
>> + * will be aligned to a cluster boundary, and the newly allocated area will be
>> + * zeroed.
>> + */
>> +static int realloc_refcount_array(BDRVQcowState *s, uint16_t **array,
>> +                                  int64_t *size, int64_t new_size)
>> +{
>> +    /* Round to clusters so the array can be directly written to disk */
>> +    size_t old_byte_size = ROUND_UP(refcount_array_byte_size(s, *size),
>> +                                    s->cluster_size);
>> +    size_t new_byte_size = ROUND_UP(refcount_array_byte_size(s, new_size),
>> +                                    s->cluster_size);
>> +    uint16_t *new_ptr;
>> +
>> +    if (new_byte_size <= old_byte_size) {
>> +        *size = new_size;
>> +        return 0;
>> +    }
> Why not realloc the array to the new smaller size? ...

Because such a call will actually never happen. I could replace this if 
() by assert(new_byte_size >= old_byte_size); if (new_byte_size == 
old_byte_size), but as I said before, I'm not a friend of assertions 
when the code can deal perfectly well with the "unsupported" case.

Max

>> +
>> +    assert(new_byte_size > 0);
>> +
>> +    new_ptr = g_try_realloc(*array, new_byte_size);
>> +    if (!new_ptr) {
>> +        return -ENOMEM;
>> +    }
>> +
>> +    memset((void *)((uintptr_t)new_ptr + old_byte_size), 0,
>> +           new_byte_size - old_byte_size);
> ...we just need to skip the memset in when new_byte_size is smaller
> than old_byte_size.