Re: [Qemu-devel] [PATCH v2] snapshot: use local variable to bdrv_pwrite_sync L1 table

[Max Reitz <mreitz@redhat.com>]

On 2014-10-21 at 13:34, Zhang Haoyu wrote:
> Use local variable to bdrv_pwrite_sync L1 table,
> needless to make conversion of cached L1 table between
> big-endian and host style.
>
> Signed-off-by: Zhang Haoyu <zhanghy@sangfor.com>
> ---
> v1 -> v2:
>   - remove the superflous assignment, l1_table = NULL;
>   - replace 512 with BDRV_SECTOR_SIZE, and align_offset with ROUND_UP
>   - remove needless check of if (l1_table) before g_free(l1_table)
>
>   block/qcow2-refcount.c | 24 +++++++-----------------
>   1 file changed, 7 insertions(+), 17 deletions(-)
>
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 2bcaaf9..29a916a 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -881,14 +881,17 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>   {
>       BDRVQcowState *s = bs->opaque;
>       uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2;
> -    bool l1_allocated = false;
>       int64_t old_offset, old_l2_offset;
>       int i, j, l1_modified = 0, nb_csectors, refcount;
>       int ret;
>   
>       l2_table = NULL;
> -    l1_table = NULL;
>       l1_size2 = l1_size * sizeof(uint64_t);
> +    l1_table = g_try_malloc0(ROUND_UP(l1_size2, BDRV_SECTOR_SIZE));

I'm sorry, but I just now realized that we don't even need the 0 variant 
here. We are never accessing any element beyond l1_table[l1_size - 1], 
and all elements from 0 until l1_size - 1 are overwritten by 
bdrv_pread() or memcpy(). Therefore we can simply use 
qemu_try_blockalign(bs->file, l1_size2) here.

(and we should be using qemu_{try_,}blockalign() instead of 
g_{try_,}malloc{0,}() whenever possible for performance reasons)

> +    if (l1_size2 && l1_table == NULL) {
> +        ret = -ENOMEM;
> +        goto fail;
> +    }
>   
>       s->cache_discards = true;
>   
> @@ -896,13 +899,6 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>        * l1_table_offset when it is the current s->l1_table_offset! Be careful
>        * when changing this! */
>       if (l1_table_offset != s->l1_table_offset) {
> -        l1_table = g_try_malloc0(align_offset(l1_size2, 512));
> -        if (l1_size2 && l1_table == NULL) {
> -            ret = -ENOMEM;
> -            goto fail;
> -        }
> -        l1_allocated = true;
> -
>           ret = bdrv_pread(bs->file, l1_table_offset, l1_table, l1_size2);
>           if (ret < 0) {
>               goto fail;
> @@ -912,8 +908,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>               be64_to_cpus(&l1_table[i]);
>       } else {
>           assert(l1_size == s->l1_size);
> -        l1_table = s->l1_table;
> -        l1_allocated = false;
> +        memcpy(l1_table, s->l1_table, l1_size2);
>       }
>   
>       for(i = 0; i < l1_size; i++) {
> @@ -1055,13 +1050,8 @@ fail:
>           }
>   
>           ret = bdrv_pwrite_sync(bs->file, l1_table_offset, l1_table, l1_size2);
> -
> -        for (i = 0; i < l1_size; i++) {
> -            be64_to_cpus(&l1_table[i]);
> -        }

But I realized something even more important as well: If you are 
modifying the current L1 table, you need to copy the result back to 
s->l1_table.

Sorry for having missed these in my previous review.

Max

>       }
> -    if (l1_allocated)
> -        g_free(l1_table);
> +    g_free(l1_table);
>       return ret;
>   }
>