Re: [Qemu-devel] [PATCH] Patch for shrinking qcow2 disk image

[Stefan Hajnoczi]

On Tue, Apr 01, 2014 at 02:06:10PM +0800, Jun Li wrote:
> Signed-off-by: Jun Li <address@hidden>
> 
> This patch can make sure the data still existing after shrinking. And only 
> discard the unused (guest) clusters. If shrinking to the size which stored 
> data, It will return an error and will not do any change.
> As this patch can support shrinking, so changed the func name of 
> qcow2_grow_l1_table to qcow2_truncate_l1_table.

Please follow the patch formatting conventions:

qcow2: add qcow2_truncate() shrinking support

...patch description...

Signed-off-by: ...

> -int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
> +int qcow2_truncate_l1_table(BlockDriverState *bs, uint64_t min_size,
>                          bool exact_size)
>  {
>      BDRVQcowState *s = bs->opaque;
> @@ -39,9 +39,6 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t 
> min_size,
>      int64_t new_l1_table_offset, new_l1_size;
>      uint8_t data[12];
>  
> -    if (min_size <= s->l1_size)
> -        return 0;
> -
>      if (exact_size) {
>          new_l1_size = min_size;
>      } else {
> @@ -66,7 +63,18 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t 
> min_size,
>  
>      new_l1_size2 = sizeof(uint64_t) * new_l1_size;
>      new_l1_table = g_malloc0(align_offset(new_l1_size2, 512));
> -    memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
> +
> +    /* shrinking the image */
> +    if (min_size <= s->l1_size) {
> +        if (s->l1_table[new_l1_size] != 0) {
> +            error_report("Could not shrink to this size, "
> +                        "it will destory image data");
> +            return -ENOTSUP;
> +        }
> +        memcpy(new_l1_table, s->l1_table, new_l1_size2);
> +    }
> +
> +   memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));

In the shrink case the second memcpy() overflows new_l1_table.

But the bigger issue I see with this patch is that bdrv_truncate() is
supposed to shrink the file, whether there is data present or not.
Please drop the if (s->l1_table[new_l1_size] != 0) test.

>  
>      /* write new table (align to cluster) */
>      BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
> @@ -559,7 +567,7 @@ static int get_cluster_table(BlockDriverState *bs, 
> uint64_t offset,
>  
>      l1_index = offset >> (s->l2_bits + s->cluster_bits);
>      if (l1_index >= s->l1_size) {
> -        ret = qcow2_grow_l1_table(bs, l1_index + 1, false);
> +        ret = qcow2_truncate_l1_table(bs, l1_index + 1, false);
>          if (ret < 0) {
>              return ret;
>          }
> diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
> index 2fc6320..ab16c52 100644
> --- a/block/qcow2-snapshot.c
> +++ b/block/qcow2-snapshot.c
> @@ -491,7 +491,7 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char 
> *snapshot_id)
>       * L1 table of the snapshot. If the snapshot L1 table is smaller, the
>       * current one must be padded with zeros.
>       */
> -    ret = qcow2_grow_l1_table(bs, sn->l1_size, true);
> +    ret = qcow2_truncate_l1_table(bs, sn->l1_size, true);
>      if (ret < 0) {
>          goto fail;
>      }
> diff --git a/block/qcow2.c b/block/qcow2.c
> index b9dc960..4797879 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1764,14 +1764,8 @@ static int qcow2_truncate(BlockDriverState *bs, 
> int64_t offset)
>          return -ENOTSUP;
>      }
>  
> -    /* shrinking is currently not supported */
> -    if (offset < bs->total_sectors * 512) {
> -        error_report("qcow2 doesn't support shrinking images yet");
> -        return -ENOTSUP;
> -    }
> -
>      new_l1_size = size_to_l1(s, offset);
> -    ret = qcow2_grow_l1_table(bs, new_l1_size, true);
> +    ret = qcow2_truncate_l1_table(bs, new_l1_size, true);
>      if (ret < 0) {
>          return ret;
>      }
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 0b0eac8..298d84e 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -455,7 +455,7 @@ int qcow2_pre_write_overlap_check(BlockDriverState *bs, 
> int ign, int64_t offset,
>                                    int64_t size);
>  
>  /* qcow2-cluster.c functions */
> -int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
> +int qcow2_truncate_l1_table(BlockDriverState *bs, uint64_t min_size,
>                          bool exact_size);

Please indent "bool exact_size" up to the '(':

+int qcow2_truncate_l1_table(BlockDriverState *bs, uint64_t min_size,
+                            bool exact_size);