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

[Stefan Hajnoczi]

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? ...

> +
> +    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.

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