Re: [Qemu-devel] [PATCH v2] qcow2: add update refcount table realization for update_refcount

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Mon,  1 Sep 2014 18:52:48 +0800
Jun Li <junmuzi@gmail.com> wrote:

> When every item of refcount block is NULL, free refcount block and reset the
> corresponding item of refcount table with NULL.
> 
> Signed-off-by: Jun Li <address@hidden>
> ---
> 
> The v2 do following change to modify some potential issue.
> 
>              +------- Here should start from "0".
>              |
>     for (k = 0; k < refcount_block_entries; k++) {
>         if (refcount_block[k] != cpu_to_be16(0)) {
>         ...                |                 |
>         }                  |                 |
>     }                      |                 +---- Using "0" is more safe.
>                            |
>                            +-------- This should be "k" not "++k".
> ---
>  block/qcow2-refcount.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 43665b8..63f36e6 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -586,6 +586,37 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
>          if (refcount == 0 && s->discard_passthrough[type]) {
>              update_refcount_discard(bs, cluster_offset, s->cluster_size);
>          }
> +
> +        /* When refcount block is NULL, update refcount table */
> +        if (block_index == 0) {
> +            int k = block_index;

Hi,

k = 0 is also done in the for block below...

> +            int refcount_block_entries = s->cluster_size / sizeof(uint16_t);

It's better for maintainance to count elements in an array this way:

int refcount_block_entries = s->cluster_size / sizeof(refcount_block[0]);

> +            for (k = 0; k < refcount_block_entries; k++) {
> +                if (refcount_block[k] != cpu_to_be16(0)) {
> +                    break;
> +                }
> +            }
> +
> +            if (k == refcount_block_entries) {
> +                qemu_vfree(refcount_block);
> +                /* update refcount table */
> +                unsigned int refcount_table_index;
> +                uint64_t data64 = cpu_to_be64(0);
> +                refcount_table_index = cluster_index >> (s->cluster_bits -
> +                                       REFCOUNT_SHIFT);
> +                ret = bdrv_pwrite_sync(bs->file,
> +                                       s->refcount_table_offset +
> +                                       refcount_table_index *
> +                                       sizeof(uint64_t),
> +                                       &data64, sizeof(data64));
> +                if (ret < 0) {
> +                    goto fail;
> +                }
> +
> +                s->refcount_table[refcount_table_index] = data64;
> +
> +            }
> +        }
>      }
> 
>      ret = 0;

Cheers.

-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.