Re: [Qemu-devel] [Qemu-stable] [PATCH v2] snapshot: use local variable tobdrv_pwrite_sync L1 table

[Max Reitz <mreitz@redhat.com>]

On 2014-10-21 at 15:08, 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.
>>
> OK, I will replace g_try_malloc0(ROUND_UP(l1_size2, BDRV_SECTOR_SIZE)) with
> qemu_try_blockalign(bs, ROUND_UP(l1_size2, BDRV_SECTOR_SIZE)).

You can actually omit the ROUND_UP(). There are no accesses beyond 
l1_table[l1_size - 1], if I'm not mistaken.

Max

>> (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.
>>
> My fault.
>> Sorry for having missed these in my previous review.
>>
>> Max
>>
>>>        }
>>> -    if (l1_allocated)
>>> -        g_free(l1_table);
>>> +    g_free(l1_table);
>>>        return ret;
>>>    }
>>>